Recent publications

Nat. Communication 2022

Elastic anisotropy of liquid crystal elastomers (LCEs) is typically measured at low frequencies for the applications such as soft robotics, actuators, and origami. The exploration of the elasticity and thermal conduction anisotropy at gigahertz frequencies will benefit its emerging high-frequency applications such as 5G cellular networks. 
MPIP researchers in collaboration with J. Liu and S. Yang (University of Pennsylvania), M. Ryu and J. Morikawa (Tokyo Institute of Technology) study the elasticity anisotropy of LCEs and its strain dependence at gigahertz frequencies utilizing Brillouin light spectroscopy. They found the unexpectedly lower Young’s modulus anisotropy than that measured by tensile testing, suggesting disparity between the local mesogenic orientation and the larger scale orientation of the network strands. Along with the observed directional heat conductivity, this work reveals the different length scales involved in the thermoelastic anisotropy and provides insights for programming liquid crystal elastomers on-demand for high-frequency applications.

The project is funded by ERC AdG SmartPhon (Grant No. 694977) more

ACS Appl. Nano Mater. 2022

Layered nanomaterials fascinate researchers for their mechanical, barrier, optical, and transport properties. Nacre is a biological example thereof, combining excellent mechanical properties by aligned submicron inorganic platelets and nanoscale proteinic interlayers. Mimicking nacre with advanced nanosheets requires ultra-confined organic layers when aiming at nacre-like high reinforcement fractions. MPIP researchers, in collaboration with Theresa Dörres, Kai Herrmann, Marius Schöttle, Daniel Wagner, Markus Retsch, and Josef Breu (University of Bayreuth, Bayreuth, Germany) and Olli Ikkala (Aalto University, Espoo, Finland), fabricated hybrid Bragg stack thin films consisting of alternating fluorohectorite-clay and polyethylene-glycol layers. Brillouin light spectroscopy (BLS) allowed determination of the orientation-dependent mechanical properties of the thin films, revealing an unprecedentedly high in-plane Young’s modulus of 162 GPa, which is more than five times the value obtained from tensile tests that are more susceptible to material defects than BLS. Complementary thermal measurements by lock-in thermography and the photoacoustic technique exhibited low cross-plane thermal conductivities κ_⊥ = 0.11 ~ 0.15 W m^-1 K^-1 and high thermal anisotropies κ_∥/κ_⊥ = 28 ~ 33. Conceptually, this work reveals the ultimate elastic and thermal properties of aligned layered clay nanocomposites in flaw-tolerant conditions. It paves the way to unraveling the complexity of designing hybrid stacks, and could eventually lead to combined molecular and nanostructured design approaches that allow for an independent adjustment of the mechanical and thermal properties.

This project is funded by ERC SmartPhon (No. 694977). more

ACS Photonics. 2022

Refractive δn index profiles (at the entrance and normal to the beam), propagation beam (left) and phase images (simulation and experiment) for self-focusing case (positive case), self-defocusing case (negative case) and co-existence of two cases (solvent mixtures). Laser propagation axis: left to right.

MPIP researcher, in collaboration with A.Bogris, N.Burger and B Loppinet (Institute of Electronic Structure and Laser ,Heraklion Greece) and K.Makris (University of Crete, Heraklion, Greece), investigated experimentally and theoretically the pattern formation in polymer binary solutions by low power visible light. DOI: https://doi.org/10.1021/acsphotonics.1c01917

We demonstrate the presence of two nonlinearities of opposite effects in the same photo-reactive polybutadiene (PB) solutions. Depending of the solvent, these present either self-focusing or defocusing nonlinearity. Both responses are local in space and nonlocal in time with time integrating response, but with different kinetics. In ternary PB solutions in two good solvents, specific light propagation and formation of unique refractive index patterns are realized and well reproduced by the model of coexistence of self-focusing and defocusing optical responses. This supports the hypothesis that the two nonlinearities have different physico-chemical origin and can coexist when mixture of solvents are used. We expect such complex response could be engineered in other polymeric materials with the requirement of two independent mechanisms leading to opposite local change of refractive index. Such novel type of highly nonlinear polymer medium may lead to complex patterning and novel lithographic techniques utilizing the large induced refractive index contrast, δn change compared to other nonlinear materials, e.g., semiconductor crystals. Recently, the self-focusing response was utilized by some of the present authors to create low loss deformable optical fiber interconnects. https://doi.org/10.1364/OL.435052
About the Project
This project is partially funded by ERC SmartPhon (No. 694977).
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Journal of Physics D: Applied Physics, 2022

Phononic crystals (PnCs) are capable to manipulate the flow of elastic energy through their periodic structures and emerge as a promising field. Thanks to the advances in microfabrication technologies and developments of multifunctional materials, the engineering of periodic structures moves forward to the nanometer scale. Hence, the relevant frequencies of elastic waves are pushed toward the gigahertz regime where strong photon-phonon interactions trigger the applications of PnCs towards information and communication technologies.

MPIP researcher in collaboration with Yu Cang (Tongji University), Yabin Jin, (Tongji University), and Bahram Djafari-Rouhani (University of Lille, Lille, France) reviewed the experimental achievements on hypersonic PnCs involving microfabrication technologies to realize the desired structures and characterization of their band structures for unraveling phonon propagation modulation. Some application-orientated research directions were proposed in terms of advances in fabrication and characterization technologies and the development of electro-optomechanical systems.

This work was supported by the ERC AdG SmartPhon (Grant 694977), Shanghai Pujiang Program (Grant No. 20PJ1413800) and the National Natural Science Foundation of China (Grant No. 12102304). more

Phys. Rev. Lett. 2022

MPIP researchers, in collaboration with Hojin Kim and Eric M. Furst (University of Delaware, Delaware, USA), Maria Secchi and Maurizio Montagna (University of Trento, Trento, Italy), and Bahram Djafari-Rouhani (University of Lille, Lille, France), investigated the acoustic eigenmodes of dumbbell-shaped nanoparticles by Brillouin light scattering experiments and theoretical calculations. The results reveal the frequencies, mode shapes, and Brillouin activities of the characteristic modes of this unique type of building blocks for achieving tunable, anisotropic phononic crystals. (Physical Review Letters. DOI: https://doi.org/10.1103/PhysRevLett.128.048003)

The vibrational eigenmodes of dumbbell-shaped polystyrene nanoparticles are recorded by Brillouin light spectroscopy (BLS), and the full experimental spectra are calculated theoretically. Different from spheres with a degeneracy of (2l + 1), with l being the angular momentum quantum number, the eigenmodes of dumbbells are either singly or doubly degenerate owing to their axial symmetry. The BLS spectrum reveals a new, low-frequency peak which is attributed to the out-of-phase vibration of the two lobes of the dumbbell. The quantization of acoustic modes in these molecule-shaped dumbbell particles evolves from the primary colloidal spheres as the separation between the two lobes increases. The elucidation of the vibration eigenmodes of the dumbbell-shaped nanoparticles and the well-controlled tunability of their geometric parameters (i.e., D₁, D₂, d₁₂) are envisaged to promote the development of a wide range of DB-based phononic crystals with direction-dependent, tunable bandgaps, which could have potential applications in areas such as nanomaterials and on-chip devices by providing more degrees of freedom in wave-based filtering and computing.

About the Project
This project was funded by ERC SmartPhon (No. 694977). more

Macromolecules 2021

BLS vibration spectra of silica nanoparticles bearing different polystyrene (PS) adsorbed layers controlled by the annealing, t_ann , time. The splitting of the fundamental (1,2) mode increases with  PS thickness. SEM of nanoparticles bearing a PS adsorbed layer before (top) and after (bottom) heating
MPIP researcher in collaboration with D. Cangialosi (Centro de Física de Materiales, San Sebastián, Spain) and K.Randazzo,, B. Zuo, D. Priestley (Chemical and Biological Engineering, Princeton University,USA) developed techniques to determine the glass transition and elasticity of thin Interfacially and irreversibly adsorbed polystyrene layers atop of silica nanoparticles.

Irreversible adsorption at polymer/substrate interfaces influences glassy properties in thin films. However, consideration has yet to be extended to the nanocomposite geometry wherein a large interfacial area and high processing temperatures afford for irreversible adsorption at the polymer/nanoparticle interface. Here, we present an approach for directly measuring the site-specific glassy properties and the elasticity at the polystyrene (PS)-adsorbed layer interface in- and the elasticity of-PS-silica model nanocomposites. We measured a depressed compared to the bulk PS T_g, glass transition temperature (T_g) via fluorescence and the nanoparticle fundamental elastic vibration sensitively depending on the PS adsorbed layers atop silica nanoparticles via Brillouin light spectroscopy (BLS). By measuring the thickness via transmission electron microscopy, the drop of T_g was observed below about 10nm PS adsorbed layers, whereas the interactions between nanoparticles significantly increased with PS thickness. Our results provide compelling evidence that adsorbed layer formation within polymer nanocomposites can have a profound impact on local interfacial properties. more

Advanced Functional Materials 2021.

MPIP researchers published a paper on meniscus-guided coating of organic semiconductors, which demonstrates how optimized charge transport properties can be achieved by controlling the fluid dynamics and crystallization.

The effects of the casting speed and solute concentration on the crystallization of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) during meniscus-guided coating (MGC) are investigated, and three morphological subregimes with increasing casting speed are identified: I) an isotropic domain-like structure; II) unidirectionally aligned crystalline bands; and III) a corrugated dendritic morphology. Interestingly, increasing the solute concentration does not affect these morphologies but merely the associated transition velocities. Numerical simulation of both the fluid dynamics in the coating bead and the crystallization itself not only explains these morphological trends but also the decrease in the width of the crystalline bands of morphology II with the casting speed. We demonstrate that the latter provides an optimal processing window for organic field-effect transistors, with minimized charge trapping, maximized on/off ratio, and reliability factor. more

Scientific Reports 2021

Hypersonic phononic bandgap structures confine acoustic vibrations whose wavelength is commensurate with that of light, and have been studied using either time- or frequency-domain optical spectroscopy. Pulsed pump-probe lasers are the preferred instruments for characterizing periodic multilayer stacks from common vacuum deposition techniques, but the detection mechanism requires the injected sound wave to maintain coherence during propagation. Beyond acoustic Bragg mirrors, frequency-domain studies using a tandem Fabry-Perot interferometer (TFPI) find dispersions of two- and three-dimensional phononic crystals (PnCs) even for highly disordered samples, but with the caveat that PnCs must be transparent. To address this problem, we demonstrated a hybrid technique for overcoming the limitations that time- and frequency-domain approaches exhibit separately: We injected coherent phonons into a non-transparent PnC using a pulsed laser and acquired the acoustic transmission spectrum on a TFPI, where pumped appeared alongside spontaneously excited (i.e. incoherent) phonons. Choosing a metallic Bragg mirror for illustration, we determined the bandgap and compared with conventional time-domain spectroscopy, finding resolution of the hybrid approach to match that of a state-of-the-art asynchronous optical sampling setup. Thus, the hybrid pump-probe technique was found to retain key performance features of the established one and going forward will likely be preferred for disordered samples. more

Macromolecules 2021

MPIP researcher in collaboration with L Gury, S. Kamble, D. Parisiand D. Vlassopoulos (IESL-FORTH, Greece) and J. Zhang, J. Lee, A. Abdullah, K. Matyjaszewski a nd  M.R. Bockstaller (CMU, Pittsburgh, PA  USA) found thatthe second virial coefficient, A₂ displays an unexpected crossover behavior from the anticipated positive to unexpected negative values in a good solvent for the polymer grafts.


Understanding the effect of polymer brush architecture on particle interactions in solution is a requisite to enable the development of functional materials based on self-assembled polymer-grafted nanoparticles (GNP). Static and dynamic light scattering of polystyrene-grafted silica particle solutions in toluene reveals that the pair-interaction potential, inferred from the second virial coefficient, A₂, is strongly affected by the grafting density,σ, and degree of polymerization, N, of tethered chains. In the limit of intermediate σ (~0.3-0.6 nm^-2) and high N, A₂ is positive and increases with N in agreement with the good solvent conditions. In contrast, for high σ > 0.6 nm^-2 and low N, A₂ displays an unexpected reversal to negative values, thus indicating poor solvent conditions. These findings are rationalized by means of a simple analysis based on a coarse-grained brush potential which balances the attractive core-core interactions and the excluded volume interactions imparted by the polymer grafts.. The effect of grafting density also illustrates the opportunity to tailor physical properties of hybrid materials by altering geometry (or architecture) rather than variation of the chemical composition.


About the Project: This project is funded by ERC SmartPhon (No. 694977). more

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Sci. Rep. 2021

MPIP researchers in collaboration with Dr .D. Navarro-Urrios snd Prof. C. M. Sotomayor-Torres (ICN2, Barcelona, Spain), Dr.  B. Graczykowski (MPIP and Adam Mickiewicz University, Poznan, Poland) demonstrated the working principle of optomechanical crystal cavities (OMC) operating under ambient conditions as a sensor of submicron particles ( Sci. Rep. 2021, 11 (1), 7829). Optomechanical crystal cavities (OMC) have rich perspectives for detecting and indirectly analysing biological particles, such as proteins, bacteria and viruses. In this work we demonstrate the working principle of OMCs operating under ambient conditions as a sensor of submicrometer particles by optically monitoring the frequency shift of thermally activated mechanical modes. The resonator has been specifically designed so that the cavity region supports a particular family of low modal-volume mechanical modes, commonly known as -pinch modes-. These involve the oscillation of only a couple of adjacent cavity cells that are relatively insensitive to perturbations in other parts of the resonator. The eigenfrequency of these modes decreases as the deformation is localized closer to the centre of the resonator. Thus, by identifying specific modes that undergo a frequency shift that amply exceeds the mechanical linewidth, it is possible to infer if there are particles deposited on the resonator, how many are there and their approximate position within the cavity region. OMCs have rich perspectives for detecting and indirectly analysing biological particles, such as proteins, viruses and bacteria. About the Project This project is partially funded by ERC SmartPhon (No. 694977). more

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J. Coll. Interf. Science (2020)

Colloidal crystals realized by self-assembled polymer nanoparticles have prominent attraction as a plat-form for various applications from assembling photonic and phononic crystals, acoustic metamaterials tocoating applications. However, the fragility of these systems limits their application horizon. In this workthe uniform mechanical reinforcement and tunability of 3D polystyrene colloidal crystals by means ofcold soldering are reported. This structural strengthening is achieved by high pressure gas (N2or Ar) plas-ticization at temperatures well below the glass transition. Brillouin light scattering is employed to mon-itor in-situ the mechanical vibrations of the crystal and thereby determine preferential pressure,temperature and time ranges for soldering,i.e.formation of physical bonding among the nanoparticleswhile maintaining the shape and translational order. This low-cost method is potentially useful for fab-rication and tuning of durable devices including applications in photonics, phononics, acoustic metama-terials, optomechanics, surface coatings and nanolithography. more

Physical Review B (2019)

We present the spontaneous Brillouin light scattering (BLS) under simultaneous one-dimensional confinementof sound and light and show that the photon-phonon coupling results from nontrivial interplay of the photoelasticand moving-interface effects. We reveal two types of BLS self-canceling: governed by mode symmetry anddriven by destructive interference of the two effects. We show that the latter can be adjusted by the lightpolarization and phonon wave number. Furthermore, we present a measurement of the shear-horizontal waves inthin membranes. more

Nano Lett. (2019)

The linear elastic properties of isotropicmaterials of polymer tethered nanoparticles (NPs) areevaluated using noncontact Brillouin light spectroscopy.While the mechanical properties of dense brush materialsfollow predicted trends with NP composition, a surprisingincrease in elastic moduli is observed in the case of sparselygrafted particle systems at approximately equal NPfillingratio. Complementary molecular dynamics simulations revealthat the stiffening is caused by the coil-like conformations ofthe grafted chains, which lead to stronger polymer−polymerinteractions compared to densely grafted NPs with shortchains. Our results point to novel opportunities to enhancethe physical properties of composite materials by the strategicdesign of the“molecular architecture”of constituents to benefit from synergistic effects relating to the organization of thepolymer component. more

Phys. Rev. Mater (2018) more

Nanotechnology (2018) more