Giuseppe M. Paternò

Synthesis of Dibenzo[hi,st]ovalene and Its Amplified Spontaneous Emission in a Polystyrene Matrix

A large number of graphene molecules, or large polycyclic aromatic hydrocarbons (PAHs), have been synthesized and display various optoelectronic properties. Nevertheless, their potential for application in photonics has remained largely unexplored. Herein, we describe the synthesis of a highly luminescent and stable graphene molecule, namely a substituted dibenzo[hi,st]ovalene (DBO 1), with zigzag edges and elucidate its promising optical-gain properties by means of ultrafast transient absorption spectroscopy. Upon incorporation of DBO into an inert polystyrene matrix, amplified stimulated emission can be observed with a relatively low power threshold (ca. 60 μJ cm-2 ), thus highlighting its high potential for lasing applications.

Near-infrared emitting single squaraine dye aggregates with large Stokes shifts

The study of supramolecular interactions and aggregation behaviour of functional materials is of great importance to tune and extend their spectral sensitivity and, hence, improve the optoelectronic response of related devices. In this study, we resolve spatially and spectrally the absorption and emission features of squaraine aggregates by means of confocal microscopy and absorption/photoluminescence spectroscopy. We observe that the aggregate affords both a broad absorption spectrum (centred at 1.85 eV and extending to 1.55 eV), likely originated by a dye configuration with allowed J- and H-arrangements, and a strong and relatively narrow emission in the near-infrared (NIR) part of the spectrum (centred at 1.59 eV), with a remarkable Stokes shift of 0.26 eV that is among the largest exhibited by squaraine dyes. These peculiarities would be beneficial for extending the spectral sensitivity of photovoltaic/light-emitting diodes and electrochemical cells, and extremely appealing for possible applications of these aggregates as NIR fluorescent probes in biomedical applications.

Spectroscopic investigation of squaraine dyes

We report a study on the excited state dynamics of two symmetric squaraine dyes, carrying different side-groups attached to the squaric ring. By means of UV-VIS absorption and time-resolved fluorescence spectroscopies, we found that the photodynamic of these functional molecules depends strongly on both the steric and electro-donating properties of the side-group.

Solution processable and optically switchable 1D photonic structures

We report the first demonstration of a solution processable, optically switchable 1D photonic crystal which incorporates phototunable doped metal oxide nanocrystals. The resulting device structure shows a dual optical response with the photonic bandgap covering the visible spectral range and the plasmon resonance of the doped metal oxide the near infrared. By means of a facile photodoping process, we tuned the plasmonic response and switched effectively the optical properties of the photonic crystal, translating the effect from the near infrared to the visible. The ultrafast bandgap pumping induces a signal change in the region of the photonic stopband, with recovery times of several picoseconds, providing a step toward the ultrafast optical switching. Optical modeling uncovers the importance of a complete modeling of the variations of the dielectric function of the photodoped material, including the high frequency region of the Drude response which is responsible for the strong switching in the visible after photodoping. Our device configuration offers unprecedented tunability due to flexibility in device design, covering a wavelength range from the visible to the near infrared. Our findings indicate a new protocol to modify the optical response of photonic devices by optical triggers only.

Excited state photophysics of squaraine dyes for photovoltaic applications: an alternative deactivation scenario

Squaraine dyes (SQs) represent a versatile class of functional molecules with strong absorption and emission features, widely used as near-infrared sensitizers in organic and hybrid photovoltaic devices. In this context, the photodynamics of such molecules has been seen to influence dramatically the efficiency of the photogeneration process. The most accepted interpretation of excited state deactivation in SQs is represented by a trans–cis photoisomerization around a CC double bond of the polymethinic-like bridge, although such scenario does not explain satisfyingly the decay route of SQs dyes in conformational constrained systems or in highly viscous environments. Here we combine steady-state and time-resolved spectroscopic techniques with high level ab initio calculations to shed light into the photophysics of cis-locked indolenine-based SQs. Our results point towards alternative deactivation routes, possibly involving a dark state in molecules lacking central substitution and the rotation of the central substituent in the core-functionalized ones. This study reveals, for the first time, the possible presence of dark electronic states in SQs, a scenario that can be further investigated in molecules exhibiting higher emission yields than cis-locked indolenine-based SQs. Furthermore, these novel results can suggest a synthetic rationale to design dyes that permit quantitative and effective charge generation/diffusion and collection in photovoltaic diodes and, thus, enhance their efficiency.

Room-Temperature Low-Threshold Lasing From Monolithically Integrated Nanostructured Porous Silicon Hybrid Microcavities

Silicon photonics would strongly benefit from monolithically integrated low-threshold silicon-based laser operating at room temperature, representing today the main challenge toward low-cost and power-efficient electronic–photonic integrated circuits. Here we demonstrate low-threshold lasing from fully transparent nanostructured porous silicon (PSi) monolithic microcavities (MCs) infiltrated with a polyfluorene derivative, namely, poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO). The PFO-infiltrated PSiMCs support single-mode blue lasing at the resonance wavelength of 466 nm, with a line width of ∼1.3 nm and lasing threshold of 5 nJ (15 μJ/cm2), a value that is at the state of the art of PFO lasers. Furthermore, time-resolved photoluminescence shows a significant shortening (∼57%) of PFO emission lifetime in the PSiMCs, with respect to nonresonant PSi reference structures, confirming a dramatic variation of the radiative decay rate due to a Purcell effect. Our results, given also that blue lasing is a worst case for silicon photonics, are highly appealing for the development of low-cost, low-threshold silicon-based lasers with wavelengths tunable from visible to the near-infrared region by simple infiltration of suitable emitting polymers in monolithically integrated nanostructured PSiMCs.

Correction: Near-infrared emitting single squaraine dye aggregates with large Stokes shifts

Correction for ‘Near-infrared emitting single squaraine dye aggregates with large Stokes shifts’ by G. M. Paterno et al., J. Mater. Chem. C, 2017, 5, 7732–7738.