We report a method for synthesizing zinc citrate spheres at a low temperature (90 °C) under normal atmospheric pressure. The spherical structures were amorphous and had an average diameter of ∼1.7 μm. The amorphous zinc citrate spheres could be converted into crystalline ZnO nanostructures in aqueous solutions by heating at 90 °C for 1 h. By local dissolution of the zinc citrate spheres, nucleation and growth of ZnO occurred on the surfaces of the amorphous zinc citrate spheres. The morphologies and exposed crystal faces of the crystalline ZnO nanostructures (structure I: oblate spheroid; structure II: prolate spheroid; structure III: hexagonal disk; structure IV: sphere) could be controlled simply by varying the solution composition (solutions I, II, III, or IV) in which the as-prepared amorphous zinc citrate spheres were converted. The concentration of citrate anions and solution pH played a decisive role in determining the morphologies and exposed crystal faces of the crystalline ZnO nanostructures. On the basis of experimental results, we propose a plausible mechanism for the conversion of amorphous zinc citrate spheres into the variety of observed ZnO structures.

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As the pitch size of Cu lines in the back-end-of-line (BEOL) is decreased below a few tens of nanometers, resistivity exponentially increases and electromigration (EM) causes device failure. Graphene has shown promise for both problems, but graphene grown at 400 °C for the BEOL-compatible process is far from its ideal honeycomb lattice. In this report, we successfully demonstrated that graphene grown at low temperatures improves Cu resistance by 5% and increased the EM lifetime by 78 times compared to Cu-only interconnect. We proved that the resistivity gain by graphene capping is due to the improvement of the Cu surface, excluding other effects of parallel resistivity and grain boundary scattering. First-principles calculation demonstrated that the graphene edge–Cu bond can inhibit the migration of Cu vacancies, thereby improving the EM lifetime. We manipulated the graphene nanostructure to have more edge contact with Cu, which enhanced the EM lifetime by 116 times compared to Cu-only interconnect. This work systematically investigated the causes for the decrease in resistance of graphene-capped Cu and discovered key factors that contribute the improvement of the interconnect reliability.

Surface-enhanced Raman spectroscopy (SERS) has been considered a promising technique for the detection of trace molecules in biomedicine and environmental monitoring. The ideal metal nanoparticles for SERS must not only fulfill important requirements such as high near-field enhancement and a tunable far-field response but also overcome the diffusion limitation at extremely lower concentrations of a target material. Here, we introduce a novel method to produce gold nanoparticles with open eccentric cavities by selectively adapting the structure of non-plasmonic nanoparticles via acid-mediated surface replacement. Copper oxide nanoparticles with open eccentric cavities are first prepared using a microwave-irradiation-assisted surfactant-free hydrothermal reaction and are then transformed into gold nanoparticles by an acidic gold precursor while maintaining their original structure. Because of the strong near-field enhancement occurring at the mouth of the open cavities and the very rough surfaces resulting from the uniformly covered hyperbranched sharp multi-tips and the free access of SERS molecules inside of the nanoparticles without diffusion limitation, adenine, one of the four bases in DNA, in an extremely diluted aqueous solution (1.0 pM) was successfully detected with excellent reproducibility upon laser excitation with a 785-nm wavelength. The gold nanoparticles with open eccentric cavities provide a powerful platform for the detection of ultra-trace analytes in an aqueous solution within near-infrared wavelengths, which is essential for highly sensitive, reliable and direct in vivo analysis. INTRODUCTIONThere is a strong demand for trace-molecule detection techniques that are simple, rapid, highly sensitive and reproducible, spanning from diagnostics in medicine to the detection of base sequence mutation. Surface-enhanced Raman spectroscopy (SERS) could be a promising candidate for extremely sensitive molecular finger-printing techniques that fulfill these technological and detection system criteria. 1 SERS is a near-field phenomenon that relies on the intensified electric fields (E-fields) on a metal nanostructure when its localized surface plasmon resonance is excited by light. 2 These enhanced E-fields lead to a large enhancement of the Raman scattering signal. 3 Although the hot spots exhibiting these intensively localized E-fields that are usually expected between two (or multiple) noble metal nanoparticles and the sharp nanoscale tips can amplify Raman signals by 410 6 times for trace molecule detection, 4,5 they are not easily obtained. 6,7 Several attempts have been made to increase the sensitivity and reproducibility for active SERS substrates. Two-dimensional arrays of various SERS-active substrates were introduced 8,9 but were not suitable for detection in solution or in an in vivo system because of