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Inorganic nanocrystal (NC) superstructures, which exhibit unique collective properties that are different to those of both the individual NCs and bulk materials, are of much scientific and technological interest. [1][2][3][4][5] For noble-metal NCs, the collective oscillation of free electrons, that is, the so-called plasmon resonance in the superstructures, provides a feasible way to realize light concentration and manipulation on a small scale.[6] Such plasmon resonance gives rise to many potential applications of noble-metal NC superstructures in different fields, for example, optical waveguides, [7] superlensing, [8] photon detection, [9] and surface-enhanced Raman scattering (SERS). Among these applications, the SERS effect based on noble-metal NC superstructures is of particular interest because of its extraordinary advantages in the highly sensitive detection of trace chemical or biological species. [10,11] The SERS effect originates from the dramatic amplification of electromagnetic fields in the NC superstructures. When the superstructures are irradiated at the wavelength that couples with the plasmon resonance of the inner NCs, the junction regions among the adjacent NCs function as "hotspots" and the local electromagnetic fields in the superstructure are amplified.[12] As a result, the Raman scattering of the detected species located at these junctions will be remarkably enhanced. Evidently, the intensity of SERS in the superstructures is determined not only by the type, shape, and size of the single NC units, but also by the inter-NC distance and arrangement pattern. Although many reports have shown that the 1D, 2D, and 3D assemblies of noble-metal NCs can be used as efficient substrates for SERS, [13] the corresponding studies on the NC superstructures are rare because of the difficulties in synthesis of monodispersed NCs that have different shapes, as well as the controlled organization of NCs on a large scale. In order to understand and maximize the SERS effect, large-scale NC superstructures with controllable morphologies are highly desirable. Herein we report how three types of Au NCs with identical sizes but different shapes can be used as building blocks to prepare superstructures on several different substrates. We demonstrate that both the structures and morphologies of the superstructures are highly dependent on the shapes of the NC units, and furthermore, that these superstructures exhibit obvious differences in their SERS properties. Both the formation mechanism and the SERS properties of the different Au NC superstructures are explored in detail.The seed-mediated growth method was used to synthesize single-crystalline rhombic dodecahedral (RD), octahedral, and cubic Au NCs by manipulating the growth kinetics of the NCs (see the Experimental Section). As reported in our previous study, RD, octahedral, and cubic Au NCs are bounded by twelve (110) planes, eight (111) planes, and six (100) planes, respectively.[14] The three types of Au NCs, with an average size of around 70 nm, have well-def...
Mild to steep standing waves of the fundamental mode are generated in a narrow rectangular cylinder undergoing vertical oscillation with forcing frequencies of 3.15 Hz to 3.34 Hz. A precise, non-intrusive optical wave profile measurement system is used along with a wave probe to accurately quantify the spatial and temporal surface elevations. These standing waves are also simulated by a two-dimensional spectral Cauchy integral code. Experiments show that contact-line effects increase the viscous natural frequency and alter the neutral stability curves. Hence, as expected, the addition of the wetting agent Photo Flo significantly changes the stability curve and the hysteresis in the response diagram. Experimentally, we find strong modulations in the wave amplitude for some forcing frequencies higher than 3.30 Hz. Reducing contact-line effects by Photo-Flo addition suppresses these modulations. Perturbation analysis predicts that some of this modulation is caused by noise in the forcing signal through ‘sideband resonance’, i.e. the introduction of small sideband forcing can generate large modulations of the Faraday waves. The analysis is verified by our numerical simulations and physical experiments. Finally, we observe experimentally a new form of steep standing wave with a large symmetric double-peaked crest, while simulation of the same forcing condition results in a sharper crest than seen previously. Both standing wave forms appear at a finite wave steepness far smaller than the maximum steepness for the classical standing wave and a surface tension far smaller than that for a Wilton ripple. In both physical and numerical experiments, a stronger second harmonic (in time) and temporal asymmetry in the wave forms suggest a 1:2 resonance due to a non-conventional quartet interaction. Increasing wave steepness leads to a new form of breaking standing waves in physical experiments.
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