Silvia G. Motti

Solution Synthesis Approach to Colloidal Cesium Lead Halide Perovskite Nanoplatelets with Monolayer-Level Thickness Control

We report a colloidal synthesis approach to CsPbBr3 nanoplatelets (NPLs). The nucleation and growth of the platelets, which takes place at room temperature, is triggered by the injection of acetone in a mixture of precursors that would remain unreactive otherwise. The low growth temperature enables the control of the plate thickness, which can be precisely tuned from 3 to 5 monolayers. The strong two-dimensional confinement of the carriers at such small vertical sizes is responsible for a narrow PL, strong excitonic absorption, and a blue shift of the optical band gap by more than 0.47 eV compared to that of bulk CsPbBr3. We also show that the composition of the NPLs can be varied all the way to CsPbBr3 or CsPbI3 by anion exchange, with preservation of the size and shape of the starting particles. The blue fluorescent CsPbCl3 NPLs represent a new member of the scarcely populated group of blue-emitting colloidal nanocrystals. The exciton dynamics were found to be independent of the extent of 2D confinement in these platelets, and this was supported by band structure calculations.

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.

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.

Using Sum-Frequency Generation (SFG) to probe electric-fields within organic field-effect transistors

Organic Field-Effect Transistors (OFETs) have attracted much research interest due to their potential for unique applications, such as flexible electronics. The operation of OFETs depends on the charge accumulation at the interface between an organic semiconductor and a dielectric material, induced by the voltage applied at the gate electrode. Direct measurements of the electric-field distribution in an operating device are useful for proposing and validating theoretical models for OFET operation. Here we propose using the second-order nonlinear optical process of Sum-Frequency Generation vibrational spectroscopy (SFG spectroscopy) to probe the presence of an electric-field in the dielectric layer of OFETs, in a non-invasive, non-destructive and remote fashion. The OFETs were fabricated with a dielectric layer consisting of poly(methyl-methacrylate) — PMMA, and an active layer based on poly(3-hexyl thiophene) — P3HT, and SFG spectra were acquired from the channel region of operating OFETs. It was observed the appearance of vibrational bands due to carbonyl groups (∼ 1720 cm−1) of the PMMA layer, whose χ(2) were induced by the electric-field within the dielectric, similarly to a reversible poling of polymers. This phenomenon opens up the possibility of mapping the spatial charge distribution in the conducting channel using SFG microscopy in operating devices.