This paper describes a facile method for generating Au@Ag core-shell nanocubes with edge lengths controllable in the range of 13.4 to 50 nm. The synthesis involved the use of single-crystal, spherical Au nanocrystals of 11 nm in size as the seeds in an aqueous system, with ascorbic acid serving as the reductant and cetyltrimethylammonium chloride (CTAC) as the capping agent. The thickness of the Ag shells could be finely tuned from 1.2 to 20 nm by varying the ratio of AgNO 3 precursor to Au seeds. We also investigated the growth mechanism by examining the effects of seeds (capped by CTAC or CTAB) and capping agent (CTAC vs. CTAB) on both size and shape of the resultant core-shell nanocrystals. Our results clearly indicate that CTAC worked much better than CTAB as a capping agent in both the syntheses of Au seeds and Au@Ag core-shell nanocubes. We further studied the localized surface plasmon resonance properties of the Au@Ag nanocubes as a function of the Ag shell thickness. By comparing with the extinction spectra obtained from theoretical calculations, we derived a critical value around 3 nm for the shell thickness at which the plasmon excitation of the Au cores would be completely screened by the Ag shells. Moreover, these Au@Ag core-shell nanocubes could be converted into Au-based hollow nanostructures containing the original Au seeds in the interiors through a galvanic replacement reaction. KeywordsCore-shell nanocubes; seed-mediated growth; cetyltrimethylammonium chloride; surface plasmonic property Gold (Au) and silver (Ag) nanocrystals have received considerable attention for many years because of their fascinating optical properties known as localized surface plasmon resonance (LSPR),1 -4 and their widespread use in applications related to photonics, catalysis, information storage, chemical/biological sensing, and surface-enhanced Raman scattering (SERS).5 -10 In an effort to tailor their properties and thus improve their performance in * corresponding author: xia@biomed.wustl.edu. Supporting Information Available:TEM and high-resolution TEM images of CTAB-Au seeds, TEM images of the products prepared by CTAB-Au seeds, extinction spectra of Au@Ag nanospheres calculated using Mie theory, extinction spectra and TEM image of the products obtained by dissolving Au@Ag core-shell nanocubes with Fe(NO 3 ) 3 solution, definition of the thickness of Ag shells, and description of truncation in a model for Au@Ag nanocubes. This material is available free of charge via the Internet at http://pubs.acs.org. 11 -20 Parallel to these developments, people have also looked into the possibility to combine Au and Ag into one single system to provide a new handle for controlling the optical and catalytic properties. To this end, Au-Ag alloy and core-shell nanocrystals have been prepared and investigated in the context of catalysis, plasmonics, sensing, imaging, and biomedicine.21 -35 Particularly, by combining Au and Ag into a core-shell configuration, their LSPR properties can be potentially tailored and finely t...

Although deformation processes in submicron-sized metallic crystals are well documented, the direct observation of deformation mechanisms in crystals with dimensions below the sub-10-nm range is currently lacking. Here, through in situ high-resolution transmission electron microscopy (HRTEM) observations, we show that (1) in sharp contrast to what happens in bulk materials, in which plasticity is mediated by dislocation emission from Frank-Read sources and multiplication, partial dislocations emitted from free surfaces dominate the deformation of gold (Au) nanocrystals; (2) the crystallographic orientation (Schmid factor) is not the only factor in determining the deformation mechanism of nanometre-sized Au; and (3) the Au nanocrystal exhibits a phase transformation from a face-centered cubic to a body-centered tetragonal structure after failure. These findings provide direct experimental evidence for the vast amount of theoretical modelling on the deformation mechanisms of nanomaterials that have appeared in recent years.

National Natural Science Foundation of China [20725310, 20721001, 20673085, 20671079, J0630429]; National Basic Research Program of China [2007CB815303, 2009CB939804]; Fujian Province of China [2005HZ013

Graphene has many unique properties which make it an attractive material for fundamental study as well as for potential applications. In this paper, we report the first experimental study of process-induced defects and stress in graphene using Raman spectroscopy and imaging. While defects lead to the observation of defect-related Raman bands, stress causes shift in phonon frequency. A compressive stress (as high as 2.1 GPa) was induced in graphene by depositing a 5 nm SiO(2) followed by annealing, whereas a tensile stress ( approximately 0.7 GPa) was obtained by depositing a thin silicon capping layer. In the former case, both the magnitude of the compressive stress and number of graphene layers can be controlled or modified by the annealing temperature. As both the stress and thickness affect the physical properties of graphene, this study may open up the possibility of utilizing thickness and stress engineering to improve the performance of graphene-based devices. Local heating techniques may be used to either induce the stress or reduce the thickness selectively.