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Small - and nano - scale soft phononics

Periodic Reporting for period 2 - Smartphon (Small - and nano - scale soft phononics)

Reporting period: 2018-03-01 to 2019-08-31

• 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-base
In the reporting period, September 01/2016 to February 28/2019 the performed work as specified under section b (Methodology) is summarized as follows:
(1). Purchase of all approved equipment and hire of two PhD’s, two Postdocs and host a Humboldt Awardee, four visiting PhD’s (Spain, USA, Japan) and one MS (Poland).
(2).Built-up and operation of: a) the backscattering/transmission Brillouin light spectroscopy (BLS) and b) the 3ω heat conductivity technique (project 2), and build-up the stimulated hypersound technique (project 3)
(3) Fabrication:
a) High aspect ratio photoresist gratings (Fig.3d) using two-beam interference lithography (Nanoscale 2017)
b) Wrinkled stratified polymer films (project 1, Fig.8b).A structured polydimethylsiloxane (PDMS) stamp was placed onto flat polymer (PC/PMMA) multilayer films. The construction was first heated up above to 413 K (above the glass transition of PMMA) and then a pressure of 100 MPa was applied to the stamp. The films were kept for 60 min at this condition. Finally, the pressure was released and the film was rapidly cooled to the room temperature (Nanotechnology 2019).
c) Fabrication of core (silica)-shell (polyethylacrylate) colloidal crystals (project 1, Fig.4c) using melt-flow technique
d) Hybrid synthetic clay sodium fluorohectorite (Hec, [Na0.5]inter[Mg2.5Li0.5]oct[Si4]tetO10F2) and polyvinylpyrrolidone (PVP) Bragg stacks were obtained upon spray coating dilute Hec/PVP dispersions (project 1,hybrid superlattices,Fig.3) The transverse flexibility and the large aspect ratio of the clay nanosheets enforces self-assembly of the components into highly defined Bragg stack with extreme polymer confinement (gallery height smaller than the PVP chains dimension) (Science Advances aax5930 under review).
e) Self-assembled dumbbell-shaped particles in a base-centered monoclinic lattice (project 1, colloid based phononics, Fig.4). Using the field-directed self-assembly technique, dumbbell-shaped polystyrene particles led to ordered structures with axis-dependent lattice parameter. (H. Kim 92nd ACS Colloid and Surface Science Symposium, June 10-13, 2018, State College, PA)
(4).Hypersonic phononics (properties, function):
a) 1D polymer based phononics with large density impedance mismatch for wide propagation stopband for hypersonic phonons (J.Chem.Phys.2017 & Perspectives in New Journal of Physics 2016)
b) Characterization of the elastic and photoelastic parameters of a periodic array of nanowalls (gratings) (project 2, Nanoscale 2018 with theory in Applied Sciences 2018).
c) Wrinkled stratified polymer films (project 2, Fig.8b) for flexible on/off bandgap switching (Nanotechnology 2019).
d) Core (silica)-shell (polyethylacrylate) (project 2, Fig.4c) flexible and tunable colloid based phononics; tunability upon stretching (Y. Cang talk in Phononics 2017: 4th International Conference on Phononic Crystals/Metamaterials, Phonon Transport/Coupling and Topological Phononics Changsha, China, June 4-June 9, 2017)
e) Dumbbell-shaped polystyrene particles (project 2) in a base- centered monoclinic structure for direction dependent phononic bandgap with a hybridization robust to order and particle anisotropy (project 2, H. Kim L7000235 APS March Meeting, March 4-8, 2019, Boston, MA)
(5). Outside the phononic band gap region:
a) Thermomechanical behavior of polymer-tethered silica particles and their assembled films (project 1&2, Fig.4d) is distinct for low and high grafting densities (Macromolecules 2017). To disentangle the role of grafted chain conformation on mechanical enhancement of polymer tether NP materials we utilized BLS and molecular dynamics simulations (Nano Lett. 2019, DOI: 10.1021/acs.nanolett.9b00817 oral in APS March Meeting, March 4-8, 2019, Boston, MA and DFG March 31-April 04,2019, Regensburg, Germany).
b) Synthesis/characterization of polymeric colloids with thin polymer layer placed atop the core by either covalently bonded or electrostatic interactions (pro
Over the period Sept.1,2016 to Feb. 28, 2019, the progress beyond the state-of- the-art reported in seven papers relates to :
(1) Fabrication:
a) High aspect ratio photoresist gratings using two-beam interference lithography (Nanoscale 2017)
b) Wrinkled stratified PC/PMMA films using structured polydimethylsiloxane (PDMS) stamps (nano printing). (Nanotechnology 2019).
c) Hybrid synthetic clay (hectorite) and polyvinylpyrrolidone (PVP) Bragg stacks were fabricated with spray coating of dilute Hec/PVP dispersions leading to highly defined hybrid Bragg stack with extreme polymer confinement and elastic impedance contrast. (Science Advances aax5930 under review).
d) Self-assembled dumbbell-shaped particles in a base-centered monoclinic lattice. Using the field-directed self-assembly technique, dumbbell-shaped polystyrene particles led to ordered structures with axis-dependent lattice parameter. (H. Kim 92nd ACS Colloid and Surface Science Symposium, June 10-13, 2018, State College, PA)
(2) Phonon Management with Hypersonic Phononic Structures:
a) A robust phonon guiding in high aspect ratio photoresist gratings (Nanoscale 2017 with cover image). The phonon propagation in low and high aspect ratio nanowalls reveals differences in elastic and photoelastic properties related to the two-beam interference lithography fabrication. The determination of the mechanical properties in nanostructured material is of great importance of their fidelity. (Detailed FEM calculations in Applied Sciences 2018, invited article)
b) Towards boosting normal incidence Bragg gap (BG) using hybrid inorganic/polymer superlattices, density plays an in the impedance contrast (density sound velocity) (J. Chem. Phys. 2017. The width of the stopband is inherently limited due to infiltration of the stiff layer due to spinning fabrication and a new method for deposition of the stiff layer (metal) is proposed (New J. Phys.2016 perspectives).
c) In nature, many periodic structures have undulating and wrinkled patterns which can regulate optical properties. Wrinkled multilayer structures might also change their phononic behavior. Recently, it was theoretically shown that interfacial wrinkles, formed due to mechanical instability, in compressed layered structures can induce band gaps in elastic wave propagation. (https://www.technologyreview.com/g/george-fytas/). Wrinkling implemented in the flat PC/PMMA multilayers by a nano printing technique introduced a second in-plane periodicity and lifted the direction invariance for the phonon propagation in the x,y plane. This permanent wrinkling of the smooth PC/PMMA films, however, had only subtle consequences in the phonon propagation, apart from a strong photonic effect revealing the folded branches associated with the new periodicity. For the proposed instability induced phononic band gap, the materials should exhibit strong stiffening with deformation.
d) In colloid based phononic crystals, BG can be controlled by the lattice size of the crystal, temperature and matrix material. The change of the crystal symmetry, however, has not been reported in phononics. Utilizing core/shell particles (Fig.4c) assembled to fcc crystal by hot shearing, we discovered an abrupt narrowing of the BG bandwidth at 40% uniaxial strain when applied only normal to the shearing direction. Parallel to the processing direction, BG was robust to the strain up to 100%. The shaped BG is due to a phase transition from fcc to distorted monoclinic lattice at 40% uniaxial strain. The stretch-induced symmetry reduction provides new ways of tuning BG gap.(IMECE Nov.6-9,2017,Tampa,FL).
e) In the previous context, dumbbell-shaped polystyrene particles can display similar, but in the absence of external stimulus, behavior. Utilizing assembled dumbbell-shaped polystyrene particles to base-centered monoclinic ordered structures, we demonstrated the simultaneous presence of anisotropic hypersonic BG’s gaps due the axis dependent lattice parameters and
DOI: 10.1021/acs.macromol.8b01804
ACS Omega 2017, 2(12), 9127
DOI: 10.1021/acs.nanolett.9b00817
DOI: 10.1021/acs.nanolett.9b00817
Nature Materials 2016, 15, 1079
DOI: 10.1038/s41598-018-35335-1
Macromolecules 2017, 50, 8658
DOI: 10.1038/s41467-018-04854-w
Nanoscale 2017, 9, 2739
DOI: 10.1103/PhysRevMaterials.2.123605