RESEARCH HIGHLIGHTS

All Highlights


Practical challenges to extrapolating Moore’s law favour alternatives to electrons as information carriers. Two promising candidates are spin-based and all-optical architectures, the former offering lower energy consumption, the latter superior signal transfer down to the level of chip-interconnects. Polaritons—spinor quasi-particles composed of semiconductor excitons and microcavity photons—directly couple exciton spins and photon polarizations, combining the advantages of both approaches. However, their implementation for spintronics has been hindered because polariton spins can be manipulated only optically or by strong magnetic fields. Here we use an external electric field to directly control the spin of a polariton condensate, bias-tuning the emission polarization. The nonlinear spin dynamics offers an alternative route to switching, allowing us to realize an electrical spin-switch exhibiting ultralow switching energies below 0.5 fJ. Our results lay the foundation for development of devices based on the electro-optical control of coherent spin ensembles on a chip.

More information can be found in: Dreismann, A, Ohadi, H, Redondo, YDVI, Balili, R, Rubo, YG, Tsintzos, SI, Deligeorgis, G, Hatzopoulos, Z, Savvidis, PG, Baumberg, JJ. "A sub-femtojoule electrical spin-switch based on optically trapped polariton condensates", NATURE MATERIALS 15, 1074 (2016).


3D mesoporous architectures of CdS and Pt nanocrystals are demonstrated as highly effective catalysts for the hydrogen evolution reaction (HER). Photocatalytic measurements coupled with spectroscopic studies suggest that deposition of ultrasmall Pt nanoparticles enhances the H2-production activity because of variations in the band-edge positions and electron donor density of CdS nanocrystals.

More information can be found in: Ioannis Vamvasakis, Bin Liu and Gerasimos S. Armatas "Size Effects of Platinum Nanoparticles in the Photocatalytic Hydrogen Production Over 3D Mesoporous Networks of CdS and Pt Nanojunctions", Adv. Func. Mater. 26, 8062-8071 (2016).


Tightly focused ultrashort laser pulses are used in many applications including laser machining of materials, eye surgery, and nanoprocessing of biological cells. Also, the high intensities that can be achieved under such conditions enable warm dense matter studies and the observation of exciting new material states. Simulations though of such extreme systems proved to be until today a tedious and unrealistic task.

We have found a way to access these complex problems through a robust and elegant approach, which is based on transformation optics, and where we simply map the problem to one that can be solved with scalar wave equations. Our solution is expected to enable access to challenging problems that were until now practically inaccessible.

More information can be found in: V. Y. Fedorov, M. Chanal, D. Grojo, and S. Tzortzakis, "Accessing Extreme Spatiotemporal Localization of High-Power Laser Radiation through Transformation Optics and Scalar Wave Equations", Phys. Rev. Lett. 117, 043902 (2016).


Inorganic nanocrystals (NCs) of uniform size are of eminent importance for the fields of catalysis, energy storage and conversion, and electrochemical devices. Current efforts in science of nanoparticles have focused on the propensity of discrete colloidal NCs to assemble into two or three-dimensional porous architectures. This aspect becomes important, especially, in applications where mass transport-related effects are a concern, such as in catalysis, separation, and chemical sensing. In this perspective, we describe how colloidal nanocrystals can be used as functional building blocks to construct highly porous networks with large and accessible surface area. The synthesis of these mesostructured assemblies, however, is not a simple process and often requires more sophisticated and elegant processing steps. We focused particularly on the potential of a polymer templating technique for the construction of ordered mesostructured assemblies of metal oxide and metal chalcogenide NCs and concentrate on the application of these materials to catalysis.

This report is referred to in J.M. Buriak, "Up-and-Coming Perspectives: Share the Excitement of Top Early Career Researchers in Materials Chemistry", Chem. Mater. 28, 4084-4084, (2016).

From: I.T. Papadas, I. Vamvasakis, I. Tamiolakis, G.S Armatas, "Templated Self-Assembly of Colloidal Nanocrystals into Three-Dimensional Mesoscopic Structures: A Perspective on Synthesis and Catalytic Prospects", Chem. Mater. 28(9), 2886-2896, (2016).


Spider dragline silk possesses superior mechanical properties compared with synthetic polymers with similar chemical structure due to its hierarchical structure comprised of partially crystalline oriented nanofibrils. To date, silk’s dynamic mechanical properties have been largely unexplored. Here we report an indirect hypersonic phononic bandgap and an anomalous dispersion of the acoustic-like branch from inelastic (Brillouin) light scattering experiments under varying applied elastic strains. We show the mechanical nonlinearity of the silk structure generates a unique region of negative group velocity, that together with the global (mechanical) anisotropy provides novel symmetry conditions for gap formation. The phononic bandgap and dispersion show strong nonlinear strain-dependent behaviour. Exploiting material nonlinearity along with tailored structural anisotropy could be a new design paradigm to access new types of dynamic behaviour.

From: Dirk Schneider, Nikolaos Gomopoulos, Cheong Y. Koh, Periklis Papadopoulos, Friedrich Kremer, Edwin L. Thomas and George Fytas "Nonlinear control of high-frequency phonons in spider silk", Nature Materials - (2016).

Laser-plasma based THz emitters offer broad radiation bandwidth, high output pulse energy and immunity to high power damage. Various excitation wavelengths and gases were explored to seek for a stronger THz wave emission from laser-induced plasmas, whereas few attempts were made for this purpose with artificially modulated exotic wave packets. We demonstrate that abruptly autofocusing beam induced air-plasmas can give a more than 5-fold enhancement of the THz wave pulse energy compared to normal Gaussian beam induced plasmas under the same conditions and this at well controlled remote locations. This work is expected to inspire a new direction for controlling THz radiation from laser-induced plasma and pave the way to THz remote spectroscopy, which is considered critical for both homeland security and environmental monitoring.

More information can be found in: K. Liu, A. D. Koulouklidis, D. G. Papazoglou, S. Tzortzakis, and X.-C. Zhang, "Enhanced terahertz wave emission from air-plasma tailored by abruptly autofocusing laser beams", Optica 3(6), 605-608 (2016).

Tunable spin correlations are found to arise between two neighboring trapped exciton-polariton condensates which spin polarize spontaneously. We observe a crossover from an antiferromagnetic to a ferromagnetic pair state by reducing the coupling barrier in real time using control of the imprinted pattern of pump light. Fast optical switching of both condensates is then achieved by resonantly but weakly triggering only a single condensate. These effects can be explained as the competition between spin bifurcations and spin-preserving Josephson coupling between the two condensates, and open the way to polariton Bose-Hubbard ladders.

From: Ohadi, H, Redondo, YDI, Dreismann, A, Rubo, YG, Pinsker, F, Tsintzos, SI, Hatzopoulos, Z, Savvidis, PG, Baumberg, JJ. "Tunable Magnetic Alignment between Trapped Exciton-Polariton Condensates", PHYSICAL REVIEW LETTERS 116, 106403 (2016).

The mechanism of flow in glassy materials is interrogated using mechanical spectroscopy applied to model nearly hard sphere colloidal glasses during flow. Superimposing a small amplitude oscillatory motion orthogonal onto steady shear flow makes it possible to directly evaluate the effect of a steady state flow on the out-of-cage (α) relaxation as well as the in-cage motions. To this end, the crossover frequency deduced from the viscoelastic spectra is used as a direct measure of the inverse microstructural relaxation time, during flow. The latter is found to scale linearly with the rate of deformation. The microscopic mechanism of flow can then be identified as a convective cage release. Further insights are provided when the viscoelastic spectra at different shear rates are shifted to scale the alpha relaxation and produce a strain rate-orthogonal frequency superposition, the colloidal analogue of time temperature superposition in polymers with the flow strength playing the role of temperature. Whereas the scaling works well for the α relaxation, deviations are observed both at low and high frequencies. Brownian dynamics simulations point to the origins of these deviations; at high frequencies these are due to the deformation of the cages which slows down the short-time diffusion, while at low frequency, deviations are most probably caused by some mild hydroclustering.

From: Alan R. Jacob, Andreas S. Poulos, Sunhyung Kim, Jan Vermant, and George Petekidis "Convective Cage Release in Model Colloidal Glasses" Phys. Rev. Lett., 115, 218301 (2015).


Hybridization-type band gaps are known to persist, despite structural disorder, in phononic crystals; however their fabrication remains challenging for all-solid hypersonic composites. Here, the authors utilize the elastic anisotropy at the interface of polymer-tethered colloidal particles to control phonon propagation in GHz regime.

From: E. Alonso-Redondo, M. Schmitt, Z. Urbach, C. M. Hui, R. Sainidou, P. Rembert, K. Matyjaszewski, M. R. Bockstaller, and G. Fytas "A new class of tunable hypersonic phononic crystals based on polymer-tethered colloids", Nat. Commun. 6 Article number: 8309, (2015).

We report the synthesis of ion-exchangeable molybdenum sulfide chalcogel through an oxidative coupling process, using (NH4)2MoS4 and iodine. After supercritical drying, the MoSx amorphous aerogel shows a large surface area up to 370 m2/g with a broad range of pore sizes. X-ray photoelectron spectroscopic and pair distribution function analyses reveal that Mo6+ species undergo reduction during network assembly to produce Mo4+-containing species where the chalcogel network consists of [Mo3S13] building blocks comprising triangular Mo metal clusters and S22- units. The optical band gap of the brown-black chalcogel is ∼1.36 eV. The ammonium sites present in the molybdenum sulfide chalcogel network are ion-exchangeable with K+ and Cs+ ions. The molybdenum sulfide aerogel exhibits high adsorption selectivities for CO2 and C2H6 over H2 and CH4. The aerogel also possesses high affinity for iodine and mercury.

From: Kota S. Subrahmanyam, Christos D. Malliakas, Debajit Sarma, Gerasimos S. Armatas, Jinsong Wu, and Mercouri G. Kanatzidis, 'Ion-Exchangeable Molybdenum Sulfide Porous Chalcogel: Gas Adsorption and Capture of Iodine and Mercury', J. Am. Chem. Soc., 137(43), 13943-13948 (2015).


Although great progress in the synthesis of porous networks of metal and metal oxide nanoparticles with highly accessible pore surface and ordered mesoscale pores has been achieved, synthesis of assembled 3D mesostructures of metal-chalcogenide nanocrystals is still challenging. In this work we demonstrate that ordered mesoporous networks, which comprise well-defined interconnected metal sulfide nanocrystals, can be prepared through a polymer-templated oxidative polymerization process. The resulting self-assembled mesostructures that were obtained after solvent extraction of the polymer template impart the unique combination of light-emitting metal chalcogenide nanocrystals, three-dimensional open-pore structure, high surface area, and uniform pores. We show that the pore surface of these materials is active and accessible to incoming molecules, exhibiting high photocatalytic activity and stability, for instance, in oxidation of 1-phenylethanol into acetophenone. We demonstrate through appropriate selection of the synthetic components that this method is general to prepare ordered mesoporous materials from metal chalcogenide nanocrystals with various sizes and compositions.

From: Ioannis Vamvasakis, Kota S. Subrahmanyam, Mercouri G. Kanatzidis, and Gerasimos S. Armatas, "Template-Directed Assembly of Metal-Chalcogenide Nanocrystals into Ordered Mesoporous Networks" ACS Nano., 9(4), 4419-4426 (2015).


Bismuth ferrite (BiFeO3) is important multiferroic oxide material because of the unique magnetic and ferroelectric properties. Here, we synthesize for the first time highly ordered mesoporous BiFeO3 semiconductor using tartaric acid-assisted growth of BiFeO3 compound inside the pores of carbon template. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and N2 physisorption measurements reveal that the template-free material possesses three-dimensional hexagonal mesostructure with large internal BET surface area (141 m2 g-1) and narrow sized pores (ca. 4 nm). Also, the pore walls comprise single-phase BiFeO3 nanocrystals according to the high-resolution TEM, electron diffraction and magnetic experiments. The mesoporous BiFeO3 shows high activity for the photocatalytic oxygen evolution reaction (OER) under UV visible light (λ > 380 nm), affording an average oxygen evolution rate of 66 μmol h-1 g-1. We also show that the propensity of photogenerated holes for OER can be significantly enhanced when 1 wt% Au nanoparticles are deposited on the BiFeO3 surface. The Au/BiFeO3 heterostructure exerts excellent OER activity (586 μmol h-1 g-1) and long-term cycling stability, raising the possibility for the design of effective and robust OER photocatalysts.

From: Ioannis Papadas, Joseph A. Christodoulides, George Kioseoglou and Gerasimos S. Armatas "High Surface Area Ordered Mesoporous BiFeO3 Semiconductor with Efficient Water Oxidation Activity" J. Mater. Chem. A, 3, 1587-1593 (2015).