Highlights

2007

Reducing Dimensions: Wiry Metals and Skinny Oxides

1. Electronic Structure of Silver Nanowires on Cu(110)

Self-assembled nanomaterials with reduced dimensionality, or at least one dimension that is on the nanoscale, such as one-dimensional nanowires, have been studied intensively in recent years. The nanostructured materials are promising building blocks for manufacturing devices of nanoelectronics and photonics from a bottom-up approach. STM results (see Fig.1) show that the Ag nanowires grown on Cu(110) are approximately 2 nm (~12 nm) in height (width). However, the nanowires orientate with the long axis parallel to the [¯110] substrate direction and posses an anisotropic morphology with aspect ratio up to 20:1. The strong anisotropic shape of the self-organized nanowires suggests a strong difference between the Ag band structure along, and perpendicular, to the nanowires. To investigate this idea, CAMD-LSU researchers, Richard Kurtz, Yaroslav Losovyj, Phil Sprunger, and Wei Chang Zhao performed angle-resolved photoemission spectroscopy (ARPES) on Ag nanowires grown on Cu(110) at the 3m NIM beamline at the CAMD synchrotron-radiation facility.


Fig.1 Previous STM results (top) show the overall morphology of Ag nano-wires (bright protrusions).
The model (bottom) shows details nanowire’s
proposed structure.

Consistent with the STM results, our ARPES results (see Fig. 2 ) show that the valence bands within the Ag nanowire are strongly anisotropic with a clear band dispersion in the along-wire direction, but no dispersion in the across-wire direction. This strongly suggests that the valence electrons of Ag behave one-dimensionally in the lateral plane (along the wire) and have little interaction with the lattice along the across-wire direction (perpendicular to the wire).
            The ARPES studies of this system demonstrate that the electronic structure of the Ag(110) nanowire on Cu(110) considerably deviates from that of bulk Ag band structure in energy dispersion behavior and even in increased energy band number. The most obvious dispersion behavior deviation is that while the photoelectron spectra show dispersion in the vertical (or (110)) and the lateral [¯110] (or along-wire) direction (see Fig. 2. (a) and (b)), they show no dispersion in the lateral [001] (or across-wire) direction because of the limited dimension of the nanowire width (~ 12 nm in average; see Fig. 2. (c) and (d)). Therefore, the dimensionality of the band structure of the Ag(110) nanowire crystal is reduced in the vertical plane formed by the cross lines parallel to the vertical [110] and the lateral [¯110] directions. This result is in accordance with the STM results.
            The results were presented at the Physical Electronics Conference at Princeton University in June, 2006.

Fig. 2. ( a) Angle-dependent photoelectron spectra from Ag/Cu(110) nanowires (21±5 ML) taken with light beam towards the [110] direction (along-wire) at photon energy of 16 eV.; ( b) same except with photon beam towards the [001] direction (across-wire) at photon energy of 16 eV: absence of Ag-d-band dispersion in across-wire direction, contrasting to that of the along-wire direction; (c) Band structure map for the two high-symmetry directions across the (110) surface Brillouin zone, indicating band dispersion in the along-wire direction and absence of dispersion in the across-wire direction. (d) Normal emission photoelectron spectra from the same Ag/Cu(110) nanowires with photon beam towards the along-wire direction ( A [001]) at photon energy of 14 eV ~ 31 eV

2. The Electronic Structure of Ultra-thin Aluminum Oxide Film Grown on FeAl(110): A Photoemission Spectroscopy


A primary goal of this NSF award is to successfully grow and characterize reduced dimensional metals (1D nanowires and 2D sheets). A strong complementary requisite to realizing this goal is to judiciously develop substrates which, electronically, are relatively inert. The goal of CAMD-LSU researchers, Phil Sprunger and Orhan Kizilkaya, was to grow the proposed nanometals on ultra-thin oxides, wherein, because of the large bandgap, the hybridization/overlap with the underlying substrate band structure is minimized. Ultra-thin oxides serve as excellent templates because they provide a unique “insulating” substrate. Due to the nano-thickness of the oxide, electron spectroscopies (ARPES, STM, EELS) can be used without the problems associated with charging. In order to enable the subsequent growth of reduced-dimensional metals, it is necessary to probe the atomic and electronic structures of the proposed ultra-thin film oxide.

Over this last year, the electronic structure of ultra-thin aluminum oxide, grown on the FeAl(110) surface, has been investigated with angle-resolved photoemission spectroscopy. As shown on the right, our scanning tunneling microscopy studies have revealed that exposing the clean FeAl(110) surface to 1000 L of oxygen at 850o C, forms a homogeneous hexagonal oxide film with a thickness of approximately 10 Angstrom. Core levels photoemission spectra of FeAl constituents indicate that Al is the only metal species present in the oxide film. As shown below, the measured band dispersion of oxide thin film indicates a 2-D electronic structure parallel to a plane of thin film due to the limited thickness of the oxide thin films. The appearance of a peak in the anticipated band gap of bulk oxide film suggests the unique electronic structure of the 2-D oxide film. This latter observation is correlated with previous scanning tunneling microscopy results to elucidate the structure of the ultra-thin aluminia film grown on FeAl(110). The results of this research will appear in the forthcoming issue of Applied Physics Journal (2007).

STM image (70x70 nm2) of oxidized FeAl(110) surface. The surface exposed to 1000 L of O2
at 850oC. A unit mesh of oxide structure
(18.6x19.6 Angstrom) is shown with solid line.

 
                             

EDCs of Al2O3 /FeAl(110) collected along high-symmetry directions of the substrate surface Brillouin zone. Dispersion of the oxide-induced states in both directions indicate a two dimensional electronic structure.