Nanometer Size Effect Research Articles

Nanoscale surface chemistry.

We report evidence in several experiments for nanometer-size effects in surface chemistry. The evidence concerns bimetallic systems, monolayer films of Pt or Pd on W(111) surfaces. Pyramidal facets with [211] faces are formed on annealing on physical monolayer of Pt, Pd on a W(111) substrate, and facet sizes increase with annealing temperature. We used synchrotron radiation-based soft x-ray photoemission to show that monolayer films of Pt, Pd, on W "float" on the outer surface, whereas multilayer films form alloys on annealing. Acetylene reactions over bimetallic planar and faceted Pd/W surfaces exhibit size effects on the nanometer scale, that is, thermal desorption spectra of reactively formed benzene and ethylene (after acetylene adsorption) change systematically with facet size. In the second case, the decomposition of C(2)H(2) over planar and faceted Ir(210) surfaces also exhibits structure sensitivity; temperature programmed desorption of H(2) from C(2)H(2) dissociation depends on the nanoscale surface structure. Finally, we have characterized interactions of Cu with the highly ordered S(4 x 4)/W(111) surface. The substrate is a sulfur-induced nanoscale reconstruction of W(111) with (4 x 4) periodicity, having broad planar terraces (approximately 30 nm in width). Fractional monolayers of vapor-deposited Cu grow as three dimensional clusters on the S(4 x 4) surface over a wide coverage range. At low Cu coverage (< or = 0.1 ML), Cu nanoclusters nucleate preferentially at characteristic 3-fold hollow sites; we find a clear energetic preference for one type of site over others, and evidence for self-limiting growth of nanoclusters.

Ultra high strength nanofilamentary conductors: the way to reach extreme properties

To enhance the intensity of non-destructive magnetic fields with long pulse duration, reinforced conductors are needed with extremely high mechanical strength and good electrical conductivity. The ideal conductors for this application should have an action integral close to that of pure copper. An elaboration process based on cold drawing and restacking has been developed at LNCMP for this purpose. The best results have been obtained with Cu/Nb nanocomposite wires with a section of 3×10 −2 mm 2 composed of a copper matrix embedding 9×10 6 continuous parallel niobium whiskers with a diameter of 40 nm. The ultimate tensile strength is 1950 MPa at 77 K. The fundamental properties linked to the effect of nanometer size have been investigated. Nevertheless, because of their small section these conductors cannot be practically used in the winding of our magnets. Therefore, we are elaborating a new generation of optimized Cu/Nb nanostructured wires exhibiting ultra high strength in a section of 2 mm 2. The latest developments are presented. Concurrently, we are developing Cu/Ta multifilamentary conductors. Since the shear modulus of tantalum is greater than that of Nb ( μ Ta≈2 μ Nb), the Cu/Ta UTS should be enhanced. However, drawing of Cu/Ta billets leads to the formation of a macroscopic roughness at the Cu/Ta interface and to the fracture of Ta. This phenomenon is interpreted in terms of stress-driven rearrangement (Grinfeld instabilities). We have investigated some solutions to prevent its formation.

Enhanced fluorescence from Eu 3+ owing to surface plasma oscillation of silver particles in glass

We describe the increase of europium ion (Eu 3+) fluorescence owing to resonant plasma oscillation of silver (Ag) particles in silica (SiO 2) glass, which were precipitated by annealing in a reducing atmosphere sol–gel derived SiO 2 glass containing Ag + and Eu 3+ ions. Nanometer size effects of Ag particles on the increase were also investigated as a function of the reduction time. The mean particle size and the volume fraction of silver particles in glass were estimated from absorption spectra using the Mie–Drude theory modified by the electron mean free path model. The fluorescence from Eu 3+ ions when the excitation was a N 2 laser increased in the presence of silver particles of ∼4.3 nm size. From comparison of the radiative damping time of the plasma oscillation with the lifetime of Eu 3+ fluorescence we, tentatively, concluded that the induced surface plasma oscillation by incidence and the interaction between Eu 3+ and Ag particles occurred simultaneously.

STRUCTURAL TRANSFORMATION IN NANOPHASE TITANIUM DIOXIDE

Nanophase TiO 2 was prepared by a stearic acid sol–gel technique. X-ray measurements showed that the as-prepared sample is anatase TiO 2 phase with an average grain size of 10.4 nm. Raman spectroscopy was used to investigate the structural transformation from anatase-to-rutile in the nanophase TiO 2 annealed at a series of temperatures from 450 to 650°C. The results showed that the transformation took place in a wide annealing temperature range. The related mechanism to the phase transformation is discussed according to nanometer size effect.

Nanometric size effects on irradiation of tin oxide powder

A nanometric powder of tin oxide (SnO2) has been irradiated with lead ions. The same grains have been observed by Transmission Electron Microscopy (TEM) before and after irradiation at a fluence of 5×1012 Pb cm−2. The shape of largest grains strongly changes while the smallest ones disappear. This phenomenon has been explained by using the thermal spike model. It appears that the irradiation induces an increase of the internal pressure in the grains leading to their explosion. In the smallest grains, the calculated maximal temperatures exceed the boiling point so that these grains disappear.

Edge state in graphene ribbons: Nanometer size effect and edge shape dependence.

Finite graphite systems having a zigzag edge exhibit a special edge state. The corresponding energy bands are almost flat at the Fermi level and thereby give a sharp peak in the density of states. The charge density in the edge state is strongly localized on the zigzag edge sites. No such localized state appears in graphite systems having an armchair edge. By utilizing the graphene ribbon model, we discuss the effect of the system size and edge shape on the special edge state. By varying the width of the graphene ribbons, we find that the nanometer size effect is crucial for determining the relative importance of the edge state. We also have extended the graphene ribbon to have edges of a general shape, which is defined as a mixture of zigzag and armchair sites. Examining the relative importance of the edge state for graphene ribbons with general edges, we find that a non-negligible edge state survives even in graphene ribbons with less developed zigzag edges. We demonstrate that such an edge shape with three or four zigzag sites per sequence is sufficient to show an edge state, when the system size is on a nanometer scale. The special characteristics of the edge state play a large role in determining the density of states near the Fermi level for graphite networks on a nanometer scale.

Nanoscale Size Effects on Photoconductivity of Semiconducting Polymer Thin Films

Bilayer polymer photoreceptors containing a trap-free tri-p-tolylamine-doped polycarbonate (TTA:PC) layer and a conjugated polymer layer are used to study nanoscale size effects in electronic polymers. A 3-fold enhancement of photogeneration quantum efficiency is observed as the thickness of poly(2,5-pyridylenebenzobisthiazole) (PPyBT) is reduced from 100 to 10 nm. The nanometer size effects originate from the small exciton diffusion lengths and the interfacial nature of the photocarrier generation.