Biomedical subjects

F J Himpsel

Publications and source records attributed to F J Himpsel.

14 recordsLinked to original sources

PubMed2006

Experimental evidence for spin-split bands in a one-dimensional chain structure.

Gold atom chains on vicinal Si(111) surfaces exhibit an unusual doublet of half-filled bands, whose origin has remained uncertain. The splitting is identified by angle-resolved photoemission as a spin splitting induced by the spin-orbit interaction (Rashba effect), in agreement with a theoretical prediction by Sánchez-Portal, Riikonen, and Martin. This interaction leads to a characteristic pattern of avoided band crossings at a superlattice zone boundary. Two out of four crossings are avoided, with a minigap E_{G}=85 meV and a k offset of 0.05 A;{-1}.

Journal Article↗

PubMed2006

Metallic single-crystal CoSi nanowires via chemical vapor deposition of single-source precursor.

We report the synthesis, structural characterization, and electrical transport properties of free-standing single-crystal CoSi nanowires synthesized via a single-source precursor route. Nanowires with diameters of 10-150 nm and lengths of greater than 10 mum were synthesized through the chemical vapor deposition of Co(SiCl(3))(CO)(4) onto silicon substrates that were covered with 1-2 nm thick SiO(2). Transmission electron microscopy confirms the single-crystal structure of the cubic CoSi. X-ray absorption and emission spectroscopy confirm the chemical identity and show the expected metallic nature of CoSi, which is further verified by room-temperature and low-temperature electrical transport measurements of nanowire devices. The average resistivity of CoSi nanowires is found to be about 510 muOmega cm. Our general and rational nanowire synthesis approach will lead to a broad class of silicide nanowires, including those metallic materials that serve as high-quality building blocks for nanoelectronics and magnetic semiconducting Fe(1-x)Co(x)Si suitable for silicon-based spintronics.

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PubMed2006

Characterization of protein immobilization at silver surfaces by near edge X-ray absorption fine structure spectroscopy.

Ribonuclease A (RNase A) is immobilized on silver surfaces in oriented and random form via self-assembled monolayers (SAMs) of alkanethiols. The immobilization process is characterized step-by-step using chemically selective near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the C, N, and S K-edges. Causes of imperfect immobilization are pinpointed, such as oxidation and partial desorption of the alkanethiol SAMs and incomplete coverage. The orientation of the protein layer manifests itself in an 18% polarization dependence of the NEXAFS signal from the N 1s to pi* transition of the peptide bond, which is not seen for a random orientation. The S 1s to C-S sigma* transition exhibits an even larger polarization dependence of 41%, which is reduced to 5% for a random orientation. A quantitative model is developed that explains the sign and magnitude of the polarization dependence at both edges. The results demonstrate that NEXAFS is able to characterize surface reactions during the immobilization of proteins and to provide insight into their orientations on surfaces.

Absorption↗

PubMed2006

Electron-phonon interaction at the Si(111)-7 x 7 surface.

It is shown that electron-phonon interaction provides a natural explanation for the unusual band dispersion of the metallic surface states at the Si(111)-(7 x 7) surface. Angle-resolved photoemission reveals a discontinuity of the adatom band at a binding energy close to the dominant surface phonon mode at h(omega0) = 70 meV. This mode has been assigned to adatom vibrations by molecular dynamics calculations. A calculation of the spectral function for electron-phonon interaction with this well-defined Einstein mode matches the data. Two independent determinations of the electron-phonon coupling parameter from the band dispersion and from the temperature-dependent phonon broadening yield similar values of lambda = 1.09 and lambda = 1.06.

Journal Article↗

PubMed2006

Nucleobase orientation and ordering in films of single-stranded DNA on gold.

We demonstrate how the orientation and ordering of DNA bases in ultrahigh vacuum (UHV) and ambient environments can be determined using complementary spectroscopic methods. Near-edge X-ray absorption fine structure (NEXAFS) with fluorescence detection, X-ray photoelectron (XPS), and Fourier transform infrared (FTIR) spectroscopies are used to quantify the coverage, chemical composition, orientation, and ordering of thymine bases in model self-assembled monolayers of thymine homo-oligonucleotides [oligo(dT)] on gold. We find that, in monolayers of thiol-modified oligo(dT), thymine bases tend to orient parallel to the Au substrate, and this preferential orientation is significantly more pronounced in monolayers of thiolated 5-mers compared to 25-mers. We interpret this preferential orientation as a signature of significant correlations (local ordering) between individual nuleobases, which offers a way to quantify and compare nucleobase interactions in films under both ambient and UHV conditions.

DNA, Single-Stranded↗

PubMed2006

Synthesis and properties of single-crystal FeSi nanowires.

We report for the first time the chemical synthesis of free-standing single-crystal nanowires (NWs) of FeSi, the only transition-metal Kondo insulator and the host structure for ferromagnetic semiconductor Fe(x)Co(1-x)Si. Straight and smooth FeSi nanowires are produced on silicon substrates covered with a thin layer of silicon oxide through the decomposition of the single-source organometallic precursor trans-Fe(SiCl3)2(CO)4 in a simple chemical vapor deposition process. Unlike typical vapor-liquid-solid (VLS) NW growth, FeSi NWs form without the addition of metal catalysts, have no catalyst tips, and depend strongly on the surface employed. X-ray spectroscopy verifies the identity and the room-temperature metallic nature of FeSi NWs. Room-temperature electrical transport measurements using NW devices show an average resistivity of 210 micro Omega cm, similar to the value for bulk FeSi. Investigations into the low-temperature physical properties of the first one-dimensional Kondo insulator and the possible new NW growth mechanism are underway. This unique synthetic approach to FeSi NWs will be generally applicable to many other transition-metal silicides.

Crystallization↗

PubMed2004

Dipole-induced structure in aromatic-terminated self-assembled monolayers: a study by near edge x-ray absorption fine structure spectroscopy.

The structure of self-assembled monolayers presenting aromatic rings at a surface is studied by near edge x-ray absorption fine structure spectroscopy (NEXAFS). Fluorine substitution at asymmetric positions in the aromatic rings is used to generate a layer of dipoles at the surface of the monolayer. We find that fluorine substituted aromatic rings are more ordered than unsubstituted aromatic rings by a factor of two based on the polarization dependence of the lowest C 1s to pi* transition, which is associated with transitions from phenyl carbons attached to hydrogens. This result is consistent with the influence of dipole-dipole interactions and quadrupolar interactions between the aromatic groups due to the substitution of fluorine atoms. The work also serves to illustrate how subtle variations in the orientation of an end group of a self-assembled monolayer can be determined by using NEXAFS.

Journal Article↗

PubMed2003

Fractional band filling in an atomic chain structure.

A new chain structure of Au is found on stepped Si(111) which exhibits a 1/4-filled band and a pair of > or =1/2-filled bands with a combined filling of 4/3. Band dispersions and Fermi surfaces for Si(553)-Au are obtained by photoemission and compared to that of Si(557)-Au. The dimensionality of both systems is determined using a tight binding fit. The fractional band filling makes it possible to preserve metallicity in the presence of strong correlations.

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PubMed2002

Structure of quantum wires in Au/Si(557).

The structure of the Au/Si(557) surface is determined from three-dimensional x-ray diffraction measurements, which directly mandate a single Au atom per unit cell. We use a "heavy atom" method in which the Au atom images the rest of the structure. Au is found to substitute for a row of first-layer Si atoms in the middle of the terrace, which then reconstructs by step rebonding and adatoms. The structure is consistent with the 1D metallic behavior seen by photoemission.

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PubMed2002

Self-organized nanoscale pattern formation on vicinal Si(111) surfaces via a two-stage faceting transition.

We demonstrate a self-organized pattern formation on vicinal Si(111) surfaces that are miscut toward the [2;11] direction. All the patterns, consisting of a periodic array of alternating (7 x 7) reconstructed terraces and step-bunched facets, have the same periodicity and facet structure, independent of the miscut angle, while the width of the facets increases linearly with miscut angle. We attribute such unique pattern formation to a surface faceting transition that involves two transition stages: the first stage forms a stress-domain structure defining the universal periodicity; the second stage forms the low-energy facets controlling the facet width.

Journal Article↗

PubMed2001

Effect of magnetic doping on the electronic states of Ni.

Angle-resolved photoemission is used to determine the change in the electronic states of Ni induced by doping with Fe and Cr. Well-defined spin and k states are selected using high energy and k resolution combined with single crystal alloys. Iron suppresses the mean free path of minority spins only, while chromium suppresses both spins and decreases the magnetic splitting. The strong variation of these effects from one impurity to the other supports the concept of magnetic doping.

Journal Article↗

PubMed2001

Band splitting for Si(557)-Au: is it spin-charge separation?

It has been proposed that the Si(557)-Au surface exhibits spin-charge separation in a one-dimensional electron liquid. Two narrowly spaced bands are found which exhibit a well-defined splitting at the Fermi level. That is incompatible with the assignment to a spinon-holon pair in a Luttinger liquid. Instead, we propose that the two bands are associated with two nearly degenerate atomic chains, or a chain of step atoms with two broken bonds. Such an assignment explains why the surface is metallic despite an even number of electrons per unit cell.

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PubMed2000

Creation of "quantum platelets" via strain-controlled self-organization at steps.

We demonstrate, by both theory and experiment, the strain-induced self-organized formation of "quantum platelets," monolayer-thick islands of finite dimensions. They form at the early stage of heteroepitaxial growth on a substrate with regularly spaced steps, and align along the steps. In the direction perpendicular to substrate steps, the island position and spacing can be preselected through substrate miscut. Along the steps, the island size and density are controlled by self-organized growth.

Journal Article↗

PubMed2000

Electron wave function at a vicinal surface: switch from terrace to step modulation.

The Cu(111) surface state has been mapped for vicinal surfaces with variable step densities by angle-resolved photoemission. Using tunable synchrotron radiation to vary the k dependence perpendicular to the surface, as well as the (k) dependence, we find a switch between two qualitatively different regimes at a miscut of 7 degrees (17 A terrace width). For larger miscut angles the step modulation of the wave function dominates, and for smaller miscut angles the terrace modulation dominates. These observations resolve an apparent inconsistency between prior photoemission and STM results.

Journal Article↗