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Small - and nano - scale soft phononics

Periodic Reporting for period 4 - Smartphon (Small - and nano - scale soft phononics)

Reporting period: 2021-03-01 to 2021-12-31

The first recorded instance of sound (phonons) appears in the 3rd verse of Genesis. The propagation of acoustic (elastic) waves in architected matter is a generic problem that impacts material and life sciences. Phonon propagation in composite structures depends on many conversational parameters (4 per solid component) increasing further when anisotropy, confinement and interfacial effects are included in the structure design. There is therefore rich unexplored and hardly predictable fundamental science that needs foundation of high frequency phononics enabling simultaneous manipulation of hypersonic phonons and visible light. The required submicron scale organization is a ubiquitous property of soft matter that allows such fabrication of structures with manifold functionalities.Control over the phonon dispersion can impact the flow of elastic waves, strength and toughness concomitantly, and 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.
1) The advancement of a new field creates knowledge in physics and engineering and challenges material nanofabrication. This know-how is being transferred to young scientists.
2) Strong, tough and low density nanostructured materials are of paramount importance for a wide range of applications comprising microelectronic, photonics, nano-electro-mechanical systems, nanofluidics, and biomedical technologies.
3) A detailed understanding of phonon propagation in soft nanostructuresis 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.
The project addresses challenging objectives at the frontiers of the involved research: a) Develop new capabilities for phonon (elastic waves) management in architected soft matter with implementation on directional strength, heat management and optomechanics. b) Extend concepts and techniques from polymer and colloid science into the new area of phononics. c) Determine currently unknown dimension- and direction-dependent thermo-mechanical properties. d) Realize functional (photo-thermal) photonic devices based on soft matter systems.
In the reporting period, September 01/2016 to December 31/2021 the performed work comprise the tasks: (i) Implementation of five new experimental techniques and the fabrication of designed polymer and colloid based on ordered and disordered structures, hybrid Bragg stacks, supraparticle assemblies and topological (kagome-type) lattices. (ii) Record of the long phonon wavelength and band gap regions of the band structure and access to direction-dependent heat conduction. (iii) Experimental access to non-transparent phononic structures, realization of a Fano coupling in nanomembranes and fast elasticity probing by means of new pump-probe hybrid techniques. (iv) Utilization of the contact-induced stretching mode to reveal interface effects and extension of the particle vibration spectroscopy to asymmetric (dumbbell-shape) nanoparticles. (v) Optomechanical enhancement in metallic nanoparticles and use of optomechanical crystals for spatial sensing of submicron sized particles. (vi) Train young scientists in an emerging field and publish 62 articles.

The research results enabled by the project methodology and the fully equipped laboratory reveal new material properties and physics underlying the unprecedented phonon propagation in architected mater summarized as: (i) Strong elasticity and heat conduction anisotropy in nanostructured materials ranging from molecular glasses to liquid crystalline elastomers and biomimetic hybrid Bragg stacks. (ii) Identification of intrinsic symmetry determined colloid eigenfrequencies and interaction induced low frequency cluster excitations laying the foundation for the particle vibration spectroscopy. (iii) Origin of lattice (Bragg) and local resonance (hybridization) induced stopbands in phononic materials. (iv) Demonstration of the working principle of optomechanical crystals cavities as a sensor of submicrometer particles and realization of the phonon-plasmon coupling for optomechanical enhancement. (v) Phonon-photon-matter interactions in periodic structures and nanomembranes discover new material functions: enhanced photo-thermal energy conversion;Fano coupling between dilatational standing Lamb waves surface waves and photo-thermally excited coherent acoustic phonons; fast light-to-motion conversion in few-nanometer thick bare polydopamine membranes stimulated by visible light. (vi) Development of a new optical technique for fast elasticity probing in particular for cryoprotectants.
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 ((https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.202170013 Journal cover);
- a Fano resonance due to the coupling of surface (Lamb) waves with photothermally excited coherent acoustic phonons (Sci. Adv. 2020 6 : eabd4540);
- quantization of acoustic modes in molecule-shaped dumbbell particles with a new low frequency stretching-like vibration of the two lobes evolving from the primary colloidal spheres as the separation between the two lobes increases (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.048003);
- strong anisotropic thermoelasticity and large glass transition temperature increase in nacre mimetic hybrid Bragg stacks with extreme polymer confinement (<1nm) (https://onlinelibrary.wiley.com/doi/ full/10.1002/ange.201911546 with Journal cover);
- for spherical polymer-grafted nanoparticle melts, the enhanced elasticity and the anomalous dissipation with decreasing graft chain length changes the current understanding of one-component nanocomposites; a multiband hypersound filtering in 2D colloid phononic structures on thin membranes (https://doi.org/10.1021/acs.nanolett.9b05101);
- fast photoactuation of bare nanometer thick polydopamine membranes attributed to the combined action of small inertia, fast water desorption and heat conduction (DOI: 10.1021/acs.nanolett.1c03165,Journal cover).
Beyond the ERC time the fully equipped laboratory will be utilized to unravel the origin of the hybridization phononic stopband of colloid based colloid with varying impedance, realize directional dependent phonon propagation in kagome-type topological structures. An ERC proof of concept application is planned for the tunable optical absorption enhancement in soft opals, nanoscale actuators stimulated and tunable orientation-dependent thermoelasticity.
Advanced Materials