Chalcogenide Glasses Show Photoinduced Second-Harmonic Generation
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scanning tunneling microscopy (STM), the researchers examined “metallic” zigzag (θ = 0) and armchair nanotubes (θ = 30). While it has been postulated that curvature effects alter the overlap of π electron wave functions creating small energy gaps in “metallic” zigzag and chiral (0 < θ < 30) SWNTs at the Fermi energy (Ef ) and that intertube interactions can break the rotational symmetry of armchair SWNTs, there has been no experimental verification of these points. STM measurements were carried out in ultrahigh vacuum at ~5 K on SWNT samples supported on Au(111) substrates. The SWNT indices (n,m), which are related to the tube diameter and chiral angle and are typically used to describe nanotubes, were assigned from the STM images. Tunneling conductance data were obtained by tunneling spectroscopy simultaneously with the STM images, and the DOS was calculated using a π-only tight-binding calculation for the assigned indices. For the “metallic” zigzag structures, the researchers’ spectroscopy data showed previously unobservable gaplike structures at Ef that demonstrate that these tubes are small-gap semicon-
ductors and not metals. As predicted, the gap magnitude was seen to depend inversely on the square of tube radius indicating that these gaps arise from curvature in the graphene sheet. Armchair nanotube experiments were performed both on a SWNT in a bundle and on an isolated tube on a Au(111) surface. Tunneling spectra on the isolated nanotube did not show any gaps but rather behaved as true metals as expected because of the crossing of π and π* bands at Ef. However, measurements on the nanotube in a bundle showed a gaplike feature that arose due to interactions between multiple tubes; this feature, termed a pseudogap, differs from the curvature-induced gaps seen in the zigzag SWNTs, because the DOS are suppressed but not reduced entirely to zero at Ef. The measurements being carried out by Ouyang and co-workers will help clarify the nature of nanotube electronic DOS near the Fermi level and thus increase the understanding of electrical transport through “metallic” SWNTs. “The absence of gaps and pseudogaps in isolated armchair SWNTs,” said Ouyang, “indicates that through control
of the local environment of these tubes, it will be possible to maximize their conductivity, possibly as interconnects in nanoelectronics.” STEFFEN K. KALDOR
Chalcogenide Glasses Show Photoinduced Second-Harmonic Generation Researchers at the Shanghai Institute of Optics and Fine Mechanics, the Japan Science and Technology Corp., and Kyoto University have observed photoinduced stable second-harmonic generation (SHG) in chalcogenide glasses. As reported in the June 15 issue of Optics Letters, J. Qiu, J. Si, and K. Hirao optically encoded SHG in 20Ge-20As-60S (mol%) glass with nanosecond laser pulses. The glass exhibits excellent photoinduced SHG conversion efficiency and stability compared to tellurite and bismuth oxide glasses. The glass was prepared by heating a 50-g mixture of Ge, As, and S in a silica ampule in a rocking electric
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