Investigations of Shape, Material and Excitation Wavelength Effects on Field Enhancement in SERS Advanced Tips

This article, a part of the larger research project of Surface-Enhanced Raman Scattering (SERS), describes an advanced study focusing on the shapes and materials of Tip-Enhanced Raman Scattering (TERS) designated to serve as part of a novel imager device. The initial aim was to define the optimal shape of the “probe”: tip or cavity, round or sharp. The investigations focused on the effect of shape (hemi-sphere, hemispheroid, ellipsoidal cavity, ellipsoidal rod, nano-cone), and the effect of material (Ag, Au, Al) on enhancement, as well as the effect of excitation wavelengths on the electric field. Complementary results were collected: numerical simulations consolidated with analytical models, based on solid assumptions. Preliminary experimental results of fabrication and structural characterization are also presented. Thorough analyses were performed around critical parameters, such as the plasmonic metal—Silver, Aluminium or Gold—using Rakic model, the tip geometry—sphere, spheroid, ellipsoid, nano-cone, nano-shell, rod, cavity—and the geometry of the plasmonic array: cross-talk in multiple nanostructures. These combined outcomes result in an optimized TERS design for a large number of applications.

On the Stability of the rotation under the action of constant momentum Lagrangian top with perfect fluid in a resisting medium

The rotation around a fixed point of a heavy dynamically symmetric solid body with an arbitrary asymmetric cavity completely filled with an ideal in-compressible liquid is considered. The stability of a uniform rotation of a Lagrang' top with the ideal liquid in a resisting medium under condition of a given constant moment is investigated. The equation of the perturbed motion of the Lagrang' top with the ideal liquid is presented. It is proved the follow-ing: the asymptotic stability of uniform rotation for an ellipsoidal cavity will be only for a compressed ellipsoidal cavity. It has been observed that most practically important cases consider the main effect of the ideal liquid influence on the motion of a solid can be researched by means of considering only the fundamental tone of the liquid oscillation. Conditions of uniform rotation asymptotic stability in a resistive medium under the action of the Lagrange top' constant moment with an arbitrary axisymmetric cavity containing an ideal liquid are obtained. Stability conditions are derived with provisions for the main and additional tones of liquid oscillations. The heavy solid body with the fixed-point value is ex-posed to the action of a constant moment in the inertial coordinate system. Analytic and numerical investigations of the main and additional tones of liquid oscillations influence, over-turning, restoring, dissipative and constant moments on the conditions of the asymptotic stability of the uniform rotation of the Lagrange top with an ideal liquid are carried out. It is stated the following: cubic and square inequalities presented in the paper are conditions of asymptotic stability if the basic tone of liquid fluctuations will be mentioned. Stability region numerical studies have been carried out on the example of an ellipsoidal cavity. It is presented that increasing of the equatorial moment of inertia of the solid body de-creases its stability region as well as the increasing of the solid body inertia axial moment in-creases the last one.

Quasicontinuum Simulation of the Effect of Lotus-Type Nanocavity on the Onset Plasticity of Single Crystal Al during Nanoindentation

Stress concentration around nanosized defects such as cavities always leads to plastic deformation and failure of solids. We investigate the effects of depth, size, and shape of a lotus-type nanocavity on onset plasticity of single crystal Al during nanoindentation on a (001) surface using a quasicontinuum method. The results show that the presence of a nanocavity can greatly affect the contact stiffness (Sc) and yield stress (σy) of the matrix during nanoindentation. For a circular cavity, the Sc and σy gradually increase with the cavity depth. A critical depth can be identified, over which the Sc and σy are insensitive to the cavity depth and it is firstly observed that the nucleated dislocations extend into the matrix and form a y-shaped structure. Moreover, the critical depth varies approximately linearly with the indenter size, regarding the same cavity. The Sc almost linearly decreases with the cavity diameter, while the σy is slightly affected. For an ellipsoidal cavity, the Sc and σy increase with the aspect ratio (AR), while they are less affected when the AR is over 1. Our results shed light in the mechanical behavior of metals with cavities and could also be helpful in designing porous materials and structures.