2,224 research outputs found
Rayleigh surface waves propagating in (111) Si substrate decorated with Ni phononic nanostructure
The paper reports results of the Surface Brillouin Light Scattering at the
silicon (111) surface loaded with a periodic 2D nickel nanostructure.
Measurements were made for samples loaded with nanostructures of different
period (different size) but of the same height. The relation between the
nanostructure size and the velocity of surface Rayleigh waves was proved to be
nonlinear. Anisotropy of the surface Rayleigh wave velocity was compared with
the results of theoretical modelling based on the Finite Element Method
Tuning of a hypersonic surface phononic band gap using a nanoscale two-dimensional lattice of pillars
We present experimental and theoretical evidence of a phononic band gap in a hypersonic range for thermally activated surface acoustic waves in two-dimensional (2D) phononic crystals. Surface Brillouin light scattering experiments were performed on the (001) surface of silicon, loaded with a 2D square lattice of 100- or 150-nm-high aluminum pillars with a spacing of 500nm. The surface Brillouin light scattering spectra revealed a different type of surface mode, related to the modulation of the lattice structure and the mechanical eigenmodes of the pillars. The experimental data were in excellent agreement with theoretical calculations performed using the finite-element method.Peer reviewe
Two-Dimensional Phononic Crystals: Disorder Matters
The design and fabrication of phononic crystals (PnCs) hold the key to
control the propagation of heat and sound at the nanoscale. However, there is a
lack of experimental studies addressing the impact of order/disorder on the
phononic properties of PnCs. Here, we present a comparative investigation of
the influence of disorder on the hypersonic and thermal properties of
two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of
circular holes with equal filling fractions in free-standing Si membranes.
Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman
thermometry based on a novel two-laser approach are used to study the phononic
properties in the gigahertz (GHz) and terahertz (THz) regime, respectively.
Finite element method simulations of the phonon dispersion relation and
three-dimensional displacement fields furthermore enable the unique
identification of the different hypersonic vibrations. The increase of surface
roughness and the introduction of short-range disorder are shown to modify the
phonon dispersion and phonon coherence in the hypersonic (GHz) range without
affecting the room-temperature thermal conductivity. On the basis of these
findings, we suggest a criteria for predicting phonon coherence as a function
of roughness and disorder.Comment: 19 pages, 4 figures, final published version, Nano Letters, 201
A novel high resolution contactless technique for thermal field mapping and thermal conductivity determination: Two-Laser Raman Thermometry
We present a novel high resolution contactless technique for thermal
conductivity determination and thermal field mapping based on creating a
thermal distribution of phonons using a heating laser, while a second laser
probes the local temperature through the spectral position of a Raman active
mode. The spatial resolution can be as small as nm, whereas its
temperature accuracy is K. We validate this technique investigating the
thermal properties of three free-standing single crystalline Si membranes with
thickness of 250, 1000, and 2000 nm. We show that for 2-dimensional materials
such as free-standing membranes or thin films, and for small temperature
gradients, the thermal field decays as in the diffusive
limit. The case of large temperature gradients within the membranes leads to an
exponential decay of the thermal field, . The
results demonstrate the full potential of this new contactless method for
quantitative determination of thermal properties. The range of materials to
which this method is applicable reaches far beyond the here demonstrated case
of Si, as the only requirement is the presence of a Raman active mode
Influence of Surfactant-Mediated Interparticle Contacts on the Mechanical Stability of Supraparticles
[Image: see text] Colloidal supraparticles are micron-scale spherical assemblies of uniform primary particles, which exhibit emergent properties of a colloidal crystal, yet exist as a dispersible powder. A prerequisite to utilize these emergent functionalities is that the supraparticles maintain their mechanical integrity upon the mechanical impacts that are likely to occur during processing. Understanding how the internal structure relates to the resultant mechanical properties of a supraparticle is therefore of general interest. Here, we take the example of supraparticles templated from water/fluorinated oil emulsions in droplet-based microfluidics and explore the effect of surfactants on their mechanical properties. Stable emulsions can be generated by nonionic block copolymers consisting of a hydrophilic and fluorophilic block and anionic fluorosurfactants widely available under the brand name Krytox. The supraparticles formed in the presence of both types of surfactants appear structurally similar, but differ greatly in their mechanical properties. While the nonionic surfactant induces superior mechanical stability and ductile fracture behavior, the anionic Krytox surfactant leads to weak supraparticles with brittle fracture. We complement this macroscopic picture with Brillouin light spectroscopy that is very sensitive to the interparticle contacts for subnanometer-thick adsorbed layers atop of the nanoparticle. While the anionic Krytox does not significantly affect the interparticle bonds, the amphiphilic nonionic surfactant drastically strengthens these bonds to the point that individual particle vibrations are not resolved in the experimental spectrum. Our results demonstrate that seemingly subtle changes in the physicochemical properties of supraparticles can drastically impact the resultant mechanical properties
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