Surface chemistry, structure, and electronic properties from microns to the atomic scale of axially doped semiconductor nanowires.

Using both synchrotron-based photoemission electron microscopy/spectroscopy and scanning tunneling microscopy/spectroscopy, we obtain a complete picture of the surface composition, morphology, and electronic structure of InP nanowires. Characterization is done at all relevant length scales from micrometer to nanometer. We investigate nanowire surfaces with native oxide and molecular adsorbates resulting from exposure to ambient air. Atomic hydrogen exposure at elevated temperatures which leads to the removal of surface oxides while leaving the crystalline part of the wire intact was also studied. We show how surface chemical composition will seriously influence nanowire electronic properties. However, opposite to, for example, Ge nanowires, water or sulfur molecules adsorbed on the exterior oxidized surfaces are of less relevance. Instead, it is the final few atomic layers of the oxide which plays the most significant role by strongly negatively doping the surface. The InP nanowires in air are rather insensitive to their chemical surroundings in contrast to what is often assumed for nanowires. Our measurements allow us to draw a complete energy diagram depicting both band gap and differences in electron affinity across an axial nanowire p-n junction. Our findings thus give a robust set of quantitative values relating surface chemical composition to specific electronic properties highly relevant for simulating the performance of nanoscale devices.

Chiral surface plasmon polaritons on metallic nanowires.

Chiral surface plasmon polaritons (SPPs) can be generated by linearly polarized light incident at the end of a nanowire, exciting a coherent superposition of three specific nanowire waveguide modes. Images of chiral SPPs on individual nanowires obtained from quantum dot fluorescence excited by the SPP evanescent field reveal the chirality predicted in our theoretical model. The handedness and spatial extent of the helical periods of the chiral SPPs depend on the input polarization angle and nanowire diameter as well as the dielectric environment. Chirality is preserved in the free-space output wave, making a metallic nanowire a broad bandwidth subwavelength source of circular polarized photons.

New flexible toolbox for nanomechanical measurements with extreme precision and at very high frequencies.

We show that the principally two-dimensional (2D) scanning tunneling microscope (STM) can be used for imaging of 1D micrometer high free-standing nanowires. We can then determine nanowire megahertz resonance frequencies, image their top-view 2D resonance shapes, and investigate axial stress on the nanoscale. Importantly, we demonstrate the extreme sensitivity of electron tunneling even at very high frequencies by measuring resonances at hundreds of megahertz with a precision far below the angstrom scale.

Probing strain in bent semiconductor nanowires with Raman spectroscopy.

We present a noninvasive optical method to determine the local strain in individual semiconductor nanowires. InP nanowires were intentionally bent with an atomic force microscope and variations in the optical phonon frequency along the wires were mapped using Raman spectroscopy. Sections of the nanowires with a high curvature showed significantly broadened phonon lines. These observations together with deformation potential theory show that compressive and tensile strain inside the nanowires is the physical origin of the observed phonon energy variations.

Direct atomic scale imaging of III-V nanowire surfaces.

We have succeeded in direct atomic scale imaging of the exterior surfaces of III-V nanowires by scanning tunneling microscopy (STM). By using atomic hydrogen, we expose the crystalline surfaces of InAs nanowires with regular InP segments in vacuum while retaining the wire morphology. We show images with atomic resolution of the two major types of InAs wurtzite nanowire surface facets and scanning tunneling spectroscopy (STS) data. Ab initio calculations of the lowest energy surface structures and simulated STM images, agree very well with experiments.

Binomial mitotic segregation of MYCN-carrying double minutes in neuroblastoma illustrates the role of randomness in oncogene amplification.

BACKGROUND: Amplification of the oncogene MYCN in double minutes (DMs) is a common finding in neuroblastoma (NB). Because DMs lack centromeric sequences it has been unclear how NB cells retain and amplify extrachromosomal MYCN copies during tumour development. PRINCIPAL FINDINGS: We show that MYCN-carrying DMs in NB cells translocate from the nuclear interior to the periphery of the condensing chromatin at transition from interphase to prophase and are preferentially located adjacent to the telomere repeat sequences of the chromosomes throughout cell division. However, DM segregation was not affected by disruption of the telosome nucleoprotein complex and DMs readily migrated from human to murine chromatin in human/mouse cell hybrids, indicating that they do not bind to specific positional elements in human chromosomes. Scoring DM copy-numbers in ana/telophase cells revealed that DM segregation could be closely approximated by a binomial random distribution. Colony-forming assay demonstrated a strong growth-advantage for NB cells with high DM (MYCN) copy-numbers, compared to NB cells with lower copy-numbers. In fact, the overall distribution of DMs in growing NB cell populations could be readily reproduced by a mathematical model assuming binomial segregation at cell division combined with a proliferative advantage for cells with high DM copy-numbers. CONCLUSION: Binomial segregation at cell division explains the high degree of MYCN copy-number variability in NB. Our findings also provide a proof-of-principle for oncogene amplification through creation of genetic diversity by random events followed by Darwinian selection.

When the genome plays dice: circumvention of the spindle assembly checkpoint and near-random chromosome segregation in multipolar cancer cell mitoses.

BACKGROUND: Normal cell division is coordinated by a bipolar mitotic spindle, ensuring symmetrical segregation of chromosomes. Cancer cells, however, occasionally divide into three or more directions. Such multipolar mitoses have been proposed to generate genetic diversity and thereby contribute to clonal evolution. However, this notion has been little validated experimentally. PRINCIPAL FINDINGS: Chromosome segregation and DNA content in daughter cells from multipolar mitoses were assessed by multiphoton cross sectioning and fluorescence in situ hybridization in cancer cells and non-neoplastic transformed cells. The DNA distribution resulting from multipolar cell division was found to be highly variable, with frequent nullisomies in the daughter cells. Time-lapse imaging of H2B/GFP-labelled multipolar mitoses revealed that the time from the initiation of metaphase to the beginning of anaphase was prolonged and that the metaphase plates often switched polarity several times before metaphase-anaphase transition. The multipolar metaphase-anaphase transition was accompanied by a normal reduction of cellular cyclin B levels, but typically occurred before completion of the normal separase activity cycle. Centromeric AURKB and MAD2 foci were observed frequently to remain on the centromeres of multipolar ana-telophase chromosomes, indicating that multipolar mitoses were able to circumvent the spindle assembly checkpoint with some sister chromatids remaining unseparated after anaphase. Accordingly, scoring the distribution of individual chromosomes in multipolar daughter nuclei revealed a high frequency of nondisjunction events, resulting in a near-binomial allotment of sister chromatids to the daughter cells. CONCLUSION: The capability of multipolar mitoses to circumvent the spindle assembly checkpoint system typically results in a near-random distribution of chromosomes to daughter cells. Spindle multipolarity could thus be a highly efficient generator of genetically diverse minority clones in transformed cell populations.

Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna.

We investigate the coupling of a single molecule to a single spherical gold nanoparticle acting as a nanoantenna. Using scanning probe technology, we position the particle in front of the molecule with nanometer accuracy and measure a strong enhancement of more than 20 times in the fluorescence intensity simultaneous to a 20-fold shortening of the excited state lifetime. Comparisons with three-dimensional calculations guide us to decipher the contributions of the excitation enhancement, spontaneous emission modification, and quenching. Furthermore, we provide direct evidence for the role of the particle plasmon resonance in the molecular excitation and emission processes.