The group has its research focus on optical and electrical properties of colloidal semiconductor nanocrystals, on metal nanostructures, graphene, and on hybrid systems that benefit from advantageous properties of those materials. Colloidal semiconductor nanocrystals can be excellent light emitters or absorbers, and the optical properties like for example the light emission wavelength, direction and polarization can be controlled via the nanocrystal size, shape and composition.
This makes them very interesting as active material in light emitting, lasing, or photovoltaic devices, and we are exploring novel approaches for proof of principle prototypes. On the other hand, metal nanostructures are very good conductors and can strongly interact with light in the visible and infrared spectral regions due to the presence of free electrons that can perform plasmon oscillations, which opened the field of nanoplasmonics.
Recently, graphene has attracted great interest as a 2D material with very appealing conductive, plasmonic, and mechanical properties. We aim at combining the favorable properties of these materials in order to investigate complex optoelectronic systems and to pave the way for novel architectures for components in photodetectors, optical communication, photovoltaics, plasmonics, and nanoscale electronics.
Congratulations to Giulia Biffi who won the ACS Photonics prize for the best oral presentation at the Nanophotonics and Micro/Nano Optics International Conference (NANOP) 2019 in Munich.
- 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 this 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 amplify the photoluminescence emission of fluorophores placed in vicinity of the cavities. [V. Caligiuri et al., ACS Photonics 2018, 5 (6), pp 2287–2294]
- 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)
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) . Raman spectroscopy investigation of dot-in-rod core/shell nanocrystals revealed a peculiar localization of the fundamental acoustic vibration modes near the core (M. Miscuglio et al., Nano Letters 12, 2016) , which has repercussions on their ground state emission. Recently, 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 ). Layered perovskites constitute a new type of low-dimensional materials, where their peculiar optical properties allow to modify their emission color by low external pressures in the MPa range (A. Castelli et al., Advanced Materials 2018)
- Plasmonics and optoelectronics in layered 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). Another interesting platform for plasmonic systems are metal/insulator/metal (MIM) stacks, and we demonstrated the tunability and coupling of the resonances in MIMIM systems and applied them for the enhancement of the emission of a layer or perovskite nanocrystals (V. Caligiuri et al., ACS Photonics 5, 2287, 2018). In this respect, we are also interested in 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). 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).
- (Photo)conductive properties of nanocrystal films and assemblies: we have employed CdS nanocrystal films for oxygen sensing (L. Maserati et al., ACS Appl. Mater. Interf. 6, 9517, 2014), demonstrated the versatility of CuSe nanosheets for the fabrication of flexible conductive films (S. Vikulov et al., Adv. Funct. Mater. 26, 3670, 2016), and 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 expoited 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 In2Se3 nanosheets 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). We also investigated DNA templates for the fabrication of suspended nanowires on micropillar arrays (E. Miele et al., Small 11, 134, 2015).
We have micromanipulator probe stations in ambient (Karl Suss) and cryogenic (Janis Research) environment that can be coupled to various light sources and lasers spanning the spectral range from UV/VIS to NIR for photoelectrical characterization. Piezo-controlled mibots (Imina) reaching nanometer control can be used for probe contacts with optical and electron microscopes. The electrical instruments such as Keithley 2612 sourcemeters, lockin amplifiers, temperature controllers etc are controlled via a Labview platform. Our group has access to the nanochemistry and optical spectroscopy labs as well as to the clean room.
- Coordinator of the H2020-MSCA-RISE project COMPASS 691185 (2016-2020)
- EU ATTRACT project “Tailored metamaterials for Extremely High-Resolution Imaging and Sensing” (TEHRIS) (2019-2020)
- Italy-Israel bilateral project “Artificial Intelligence and Data Management for Colloidal Quantum Dot Materials Research” (AI-4-QD)
- Prof. Pingheng Tan, Chinese Academy of Sciences, Institute of Semiconductors
- Prof. Davide Comoretto, University of Genoa
- Dr. Alexander Weber-Bargioni, Lawrence Berkeley National Lab, California, USA
- Prof. P. James Schuck, Columbia University, New York, USA
- Dr. Cinzia Giannini, IC-CNR, Bari, Italy
- Prof. Iwan Moreels, University of Ghent, Belgium
Roman Krahne was appointed Guest Professor at the Institute of Semiconductors, Chinese Academy of Sciences