We introduce a unique surface-enhanced Raman scattering (SERS) substrate, which is made of hierarchical nanoarrays comprising silicon nanowires (SiNWs) decorated with silver nanoparticles (AgNPs). SiNWs arrays were produced through chemical etching of n-Si(100) wafer, while AgNPs were directly grown on SiNWs through galvanic redox reaction without any organic contamination. Solid surface extinction spectrum revealed strong surface plasmon absorption in a broad range (from 200 to 1000 nm) indicating that surface plasmon induced by the incident laser could resonate with incident and scattering photons in a broad wavelength region. The 3D arrangement of the AgNPs@SiNWs array offered significant advantages over conventional planar geometry, where the long interaction distance favored multiple plasmon resonances and strong electromagnetic enhancement effect. The substrate exhibited strong surface-enhanced effect in Raman scattering with a few percent reproducibility in Raman signals. The observed enhancement factor of the substrate was 8-10 orders of magnitude, which is several orders of magnitude higher than that from common substrates. Significantly, amounts as low as several tens of Sudan dye molecules placed on the present substrate could still display molecular structure-specific Raman bands.

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The relative stabilities of nonisomers are investigated. Twenty-two species of nitrogen cage molecules N(2n) (N6 (D(3h)), N8 (Oh), N10 (D(5h)), N12 (D(6h)), N12 (D(3d)), N16 (D(4d)), N18 (D(3h)), N20 (Ih), N24 (D(3d)), N24 (D(4h)), N24 (D(6d)), N30 (D(3h)), N30 (D(5h)), N32 (D(4d)), N36 (D(3d)), N40 (D(4h)), N42 (D(3h)), N48 (D(4d)), N48 (D(3d)), N54 (D(3h)), N56 (D(4h)), and N60 (D(3d))), which are divided into four sets, have been studied in detail. The geometries and varieties of energies are examined extensively, and NBO analysis and AIM analysis are applied to investigate the bonding properties of the cage molecules. The introducing of the concept of "layer" can well assist in explaining why one nonisomer molecule is more stable than another one. The results show that the lengths of bonds, on both sides of which are five-membered rings (referred to as pentagons), are the shortest and the orbital energies are the lowest. The nonlocalized electron numbers of orbitals, on at least one side of which is a triangle, are the greatest. Pentagons play a major role in the stability of a cage molecule, and the three-membered rings (referred to as triangles) play the second one. The layers in nitrogen cage molecules also contribute to the relative stabilities.