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Small - and nano - scale soft phononics

Periodic Reporting for period 3 - Smartphon (Small - and nano - scale soft phononics)

Reporting period: 2019-09-01 to 2021-02-28

• What is the problem/issue being addressed?
The propagation of elastic waves (acoustic waves or phonons) in architected matter is a generic problem that impacts many areas of material science. Phonon propagation in structure composite structures depends on many interrelated parameters (four for each solid component). This number increases further when anisotropy is introduced in the design of the structures. Besides the structural and elastic parameters, phononic behavior is further influenced by additional factors such as complex structural relaxation, confinement and interfacial effects. There is therefore rich unexplored and hardly predictable fundamental science that needs a supporting foundation to be established. The key quantity is the dispersion relation ω(k), that relates the frequency ω and the wave vector k of the propagating wave, in the composite matter. Engineering of ω(k) to allow elastic wave propagation only on desired frequencies , polarizations and directions (metamaterials) requires control of both structure periodicity and component dimensions and elastic parameters. Extension to high frequency phononics, to enable simultaneous manipulation of hypersonic phonons and visible light, needs organization in the submicron and nanometer scale range via self-assembly and nanofabrication. This is a ubiquitous property of soft matter (polymers, colloids) that allows such fabrication of complex hierarchical structures with tailored and manifold functionalities. Control over the phonon dispersion and the anisotropic elasticity can impact the heat transport in dielectric hybrid materials. Many important questions in this young field of small-nano scale phononics are just being raised and require new conceptual and technical approaches to address them.

• Why is it important for society?
1) The advancement of a new field creates new knowledge in physics and engineering and challenges material nanofabrication. This know-how is being transferred to young scientists (PhD, Postdoc’s and visiting scholars) working in this program. 2) Mechanical properties and the stability of nanostructured materials are of paramount importance for a wide range of applications that comprise microelectronic, photonics, nano-electro-mechanical systems, nanofluidics, and biomedical technologies. Crowding, space-filling and confinement in soft condensed matter are conditions that can have severe effect on the material properties in particular at metastable states. 3) A detailed understanding of phonon propagation in soft nanostructured media is a precondition to access fundamental concepts such as heat management and phonon-photon interactions. Heat management is increasingly being recognized as a key technology to fuel the growth of our technology-driven society. Controlling the elusive flow of heat is a complex challenge across multiple materials, length scales, and ultimately devices. It suffices to mention the wastage through heat in ICT devices is enormous (several hundred TWh, only in Germany). Strong optoacoustic effect can find applications also in devices as nanoactuators. Other applications, e.g. in nondestructive metrology, can emerge from the still unprecedented phonon-matter interactions.

• What are the overall objectives?
The project addresses challenging objectives at the frontiers of the involved research: a) Develop new capabilities for phonon (elastic waves) management in mesoscopic soft matter with implementation on heat management. The integrated approach, utilization of the flexibility, versatility and multi-functionality of soft matter, will generate the missing information for phonon band structure engineering as is necessary to tailor the dispersion relations for a particular application. b) Extend concepts and techniques from polymer and colloid science into the new area of phononics. Structure and its component materials dictate the shape of the experimental phonon dispersion relations. Soft matter-based phononics utilize the main advantages of facile processing as well as architectural and topological changes of the constituent components. c) Determine currently unknown dimension-dependent thermo-mechanical properties. The ability to pattern substrates at the nanoscale has largely evolved as a result of advances in thin film technologies and electronics; the stability of nanostructured hybrid films is of paramount importance. d) Realize functional photonic devices based on soft matter systems. Since hypersonic phononic crystals have lattice constants in the range of visible/near IR light, phoxonic materials should enable localization of both sound and light in the same regions. The transfer of the hard-matter optomechanics to soft materials taking advantage of the rich structures accessible is of great interest but it constitutes a “formidable” challenge with high physical and engineering impact.
In the reporting period, September 01/2016 to February 28/2021 the performed work comprised the implementation of the new experimental techniques, fabrication of designed polymer and colloid based ordered structures and disordered and hybrid Bragg stacks necessary to address the low phonon wavelength and gap regions of the band structure. The former regime accessed by non-contact, non-invasive, zero-strain and direction selective optical techniques revealed strong elasticity anisotropy along with anisotropic heat transport, impact of the chain conformation in polymer grafted colloids and chain-orientation in stimuli responsible polymers. The experimental phonon dispersion near the Brillouin zone besides the expected lattice symmetry dependence, verified for spherical colloidal systems, displayed strong sensitivity to surface waves (2D) and colloid architecture (3D dumbbell shape and polymer grafted).The presence of strong elastic resonances has utilized to tune the hybridization bandgap (robust to disorder), to sense interparticle interaction and phase transition, and further develop the particle vibration spectroscopy harnessing the optomechanical enhancement in metallic nanoparticles. For non-transparent phononics a new stimulated hypersound technique and a modified version for fast elasticity probing have been developed.
In addition to the delicate phonon propagation in architected materials necessary to create the missing predictive power, the research of small- and nano-scale soft phononics led to unexpected novel material functions so far: A strong resonance –enhanced photo-thermal energy conversion in transparent hybrid opals due to enhanced density of states; the coupling of surface (Lamb) waves with photothermally excited coherent acoustic phonons in Si membranes was detected revealing the Fano resonance; dumbbell colloidal crystals exhibit anisotropic phononic band gaps and the lowest frequency vibration is a stretching-like mode absent in the spherical colloids; nacre mimetic hybrid Bragg stacks with extreme polymer confinement (<1nm) revealed strong anisotropic thermoelasticity with large glass transition temperature shift through interfacial slow down. The utilization of all possible scattering configurations in the low phonon wavelength region led to the determination of the complete elasticity of spider silk correcting errors in the literature.
DOI: 10.1038/s41467-018-04854-w
Nature Materials 2016, 15, 1079
DOI: 10.1038/s41598-018-35335-1
Nanoscale 2017, 9, 2739
ACS Omega 2017, 2(12), 9127
DOI: 10.1021/acs.nanolett.9b00817
DOI: 10.1021/acs.macromol.8b01804
DOI: 10.1021/acs.nanolett.9b00817
Macromolecules 2017, 50, 8658
DOI: 10.1103/PhysRevMaterials.2.123605