Metamaterials: We develop layered Metal/Insulator/Metal structures as photonic and plasmonic cavities, and demonstrate that their resonances correspond to Epsilon-near-zero modes. [V. Caligiuri et al., Nano Letters 2019, 19, 3151-3160]. A semiclassical treatment of the optical resonances of this material system gives further insight into their physical behavior, and allows for a more facile calculation of their resonance frequencies. Coupling of multiple cavities leads to splitting of the resonances such that their frequencies can be tailored within the visible spectral range, [V. Caligiuri et al., Nanophotonics 2019] which can be employed to design Epsilon-near-zero bands [V. Caligiuri et al., Adv. Photon. Res. 2021, 2, 2100053], and to amplify the photoluminescence emission of fluorophores placed in vicinity of the cavities. [V. Caligiuri et al., ACS Photonics 2018, 5 , 2287–2294]. With the emitters inside the insulator layer of the cavity the directionality and spectral resonance profile of the cavity resonances can be transferred to the emission properties [V. Caligiuri et al., Adv. Opt. Mater. 2019, 8, 1901215]. With open cavities that allow full optical access to the emitter layer we observe strong light-matter coupling. [A. Patra et al., Adv. Opt. Mater. 2021, 9, 2101076] In collaboration with the Nicolò Maccaferri at the University of Luxembourg we demonstrated ultrafast optical switching using such Epsilon-near-zero cavities by exploiting pump-induced changes to the refractive index of the materials [J. Kuttruff et al., Communication Physics 2020, 3, 114].
Layered Metal-Halide Perovskites: These emergent materials consist of organic/inorganic bilayer architectures and demonstrate many interesting properties such as quantum confinement and strong exciton binding energies. Similar to other van-der-Waals crystals, single flake-like structures can be mechanically exfoliated and deposited on a variety of different substrates. We demonstrate that their emission can be tuned from warm white light to blue by using different ammonium molecules as organic spacers. [B. Dhanabalan et al., Adv. Mater. 2021, 33, 2008004] We study the optical and vibrational properties of two-dimensional layered perovskites by photoluminescence and Raman spectroscopy, and revealed high photoluminescence quantum yield from single flakes [Dhanabalan et al., Nanoscale 2019, 11, 8334], and the directional properties of the vibrational modes [Dhanabalan et al., ACS Nano 2020, 14, 4689]. Due to their peculiar optical properties, it is possible to modify the emission color of Lead-Bromide layered perovskites by low external pressures in the MPa range (A. Castelli et al., Advanced Materials 2018). In the low-dimensional family of the Cs2AgBiBr6 double perovskites, we discovered phase transitions that involve both organic and inorganic layers, and with density functional theory calculations we gained insight to the optical properties and revealed an extraordinarily flat conduction band. [Martìn-Garçia et al., J. Phys. Chem. Lett. 2020].
Optical properties of colloidal nanocrystal films and assemblies for LEDs, gain and lasing: Following our work on nanocrystal coffee-ring lasers (M. Zavelani et al., Laser&Photonics Reviews 6, 678, 2012), we have demonstrated optically pumped lasing from water-soluble core-shell CdSe/CdS nanocrystals (F. Di Stasio et al., Small 11, 1328, 2015) with further reduced threshold, and obtained substrate-free three-dimensional nanocrystal assemblies for color conversion by slow solvent evaporation on superhydrophobic substrates (A. Accardo et al., PPSC 32, 524, 2015). Embedding such dot-in-rod layers in polymeric Bragg reflectors enabled the fabrication of flexible cavities and lasers ( G. Manfredi et al., ACS Photonics 2017, 4, 1761; G. Manfredi et al., RCS Advances 8, 13026, 2018). Towards LEDS, we could improve the photoluminescence quantum efficiency of the emitting layer and optimize the balance of the injected charge carriers tailoring the surface ligand passivation in dot-in-rod films (P. Rastogi et al., ACS Applied Materials & Interfaces 10, 5665, 2018). Perovskite materials such as cesium lead halide nanocrystals have become a very promising material for light emitting applications, and we have tested them in LEDS(J. Shamsi et al., ACS Nano,11, 10206, 2017), for white color conversion (F. Palazon et al., Chem. Mater. 28, 2902, 2016), and studied the impact of post synthesis treatments on films from such materials on their optical and electrical properties ( F. Palazon et al., J. Mater. Chem. C 4, 9179, 2016; F. Palazon et al., ACS Applied Nano Materials, 1 (10), pp 5396, 2018). Here we also studied the overcoating of light emitting nanocrystal films by atomic-layer deposition of oxide films (M. Palei et al., ACS Applied Materials & Interfaces, 10, 22356, 2018; M. Palei et al., ACS Applied Nano Materials 2020, 3, 8167). We developed blends of „giant-shell“ nanocrystals that show ASE over a spectral range of more that 150 nm (F. Di Stasio et al., ACS Photonics 3, 2083, 2016), which makes this material interesting for broadband lasers. Together with Iwan Moreels group we demonstrated optically pumped lasing of colloidal nanocrystals under constant power excitation (J. Grim et al., Nature Nanotechnology 9, 891, 2014).
Plasmonics and two-dimensional materials: We investigated plasmonic waveguides in graphene, and developed concepts for shape-control and structuring that promise a significant enhancement of graphene plasmon propagation (M. Miscuglio et al., ACS Photonics 3, 2170, 2016). Combining graphene with two-dimensional transition-metal dichalcogenides we obtained multifunctional heterostructures for optoelectronics(A. Rossi et al., Nanoscale,10, 4332, 2018). Lateral patterning of gold metal films into fractal structures leads to multiple plasmon resonances in the visible and near-infrared region that can be exploited for broad band and spatially distributed near-field enhancement (F. De Nicola et al., ACS Photonics 5, 2418, 2018), and the combination with graphene allowed for the fabrication of metamaterial broadband photodetectors ( F. De Nicola et al., Scientific Reports 2020, 10, 1).
(Photo)conductive properties of nanocrystal films and assemblies: We investigated the formation and photoconductivity of networks of CuTe nanoplatelets in a polymeric matrix (M. Arciniegas et al., Adv. Funct. Mater. 26, 4535, 2016). We also exploited Ag2S nanocrystal films for proof of concept resistive switching memories (B. Martin-Garcia et al., Journal Materials Chemistry C, 2018). On the single nanostructure level we contacted individual In2Se3nanosheets to measure their (photo)electrical properties (G. Almeida et al., JACS 139, 3005, 2017), and we investigated the cation exchange and heterostructure formation in Cu2-xSe/CdSe nanowires (S. Dogan et al., Nature Communcations 9, 505, 2018). Following up on our previous work on octapod self-assembly into ordered chains and three dimensional structures, we studied octapod assembly insitu by transmission electron microscopy (E. Sutter et al., Nature Commun. 7, 11213, 2016), and monitored the transformation of CsPbBr3 nanoplatelets into larger nanotiles ( Z. Dang et al., Nano Letters 2020, 20,1808).
