Carlo Giansante

White-Light-Emitting Self-Assembled NanoFibers and Their Evidence by Microspectroscopy of Individual Objects

The self-assembly of a blue-emitting light-harvesting organogelator and specifically designed highly fluorescent tetracenes yields nanofibers with tunable emissive properties. In particular, under near-UV excitation, white light emission is achieved in organogels and dry films of nanofibers. Confocal fluorescence microspectroscopy demonstrates that each individual nanofiber emits white light. A kinetic study shows that an energy transfer (ET) occurs between the blue-emitting anthracene derivative and the green- and red-emitting tetracenes, while inter-tetracene ETs also take place. Moreover, microscopy unravels that the nanofibers emit polarized emission in the blue spectral region, while at wavelengths higher than 500 nm the emission is not significantly polarized.

“Darker-than-Black” PbS Quantum Dots: Enhancing Optical Absorption of Colloidal Semiconductor Nanocrystals via Short Conjugated Ligands

Colloidal quantum dots (QDs) stand among the most attractive light-harvesting materials to be exploited for solution-processed optoelectronic applications. To this aim, quantitative replacement of the bulky electrically insulating ligands at the QD surface coming from the synthetic procedure is mandatory. Here we present a conceptually novel approach to design light-harvesting nanomaterials demonstrating that QD surface modification with suitable short conjugated organic molecules permits us to drastically enhance light absorption of QDs, while preserving good long-term colloidal stability. Indeed, rational design of the pendant and anchoring moieties, which constitute the replacing ligand framework leads to a broadband increase of the optical absorbance larger than 300% for colloidal PbS QDs also at high energies (>3.1 eV), which could not be predicted by using formalisms derived from effective medium theory. We attribute such a drastic absorbance increase to ground-state ligand/QD orbital mixing, as inferred by density functional theory calculations; in addition, our findings suggest that the optical band gap reduction commonly observed for PbS QD solids treated with thiol-terminating ligands can be prevalently ascribed to 3p orbitals localized on anchoring sulfur atoms, which mix with the highest occupied states of the QDs. More broadly, we provide evidence that organic ligands and inorganic cores are inherently electronically coupled materials thus yielding peculiar chemical species (the colloidal QDs themselves), which display arising (opto)electronic properties that cannot be merely described as the sum of those of the ligand and core components.

The Dynamic Organic/Inorganic Interface of Colloidal PbS Quantum Dots.

Colloidal quantum dots are composed of nanometer-sized crystallites of inorganic semiconductor materials bearing organic molecules at their surface. The organic/inorganic interface markedly affects forms and functions of the quantum dots, therefore its description and control are important for effective application. Herein we demonstrate that archetypal colloidal PbS quantum dots adapt their interface to the surroundings, thus existing in solution phase as equilibrium mixtures with their (metal-)organic ligand and inorganic core components. The interfacial equilibria are dictated by solvent polarity and concentration, show striking size dependence (leading to more stable ligand/core adducts for larger quantum dots), and selectively involve nanocrystal facets. This notion of ligand/core dynamic equilibrium may open novel synthetic paths and refined nanocrystal surface-chemistry strategies.

Quantum-Confined and Enhanced Optical Absorption of Colloidal PbS Quantum Dots at Wavelengths with Expected Bulk Behavior

Nowadays it is well-accepted to attribute bulk-like optical absorption properties to colloidal PbS quantum dots (QDs) at wavelengths above 400 nm. This assumption permits to describe PbS QD light absorption by using bulk optical constants and to determine QD concentration in colloidal solutions from simple spectrophotometric measurements. Here we demonstrate that PbS QDs experience the quantum confinement regime across the entire near UV-vis-NIR spectral range, therefore also between 350 and 400 nm already proposed to be sufficiently far above the band gap to suppress quantum confinement. This effect is particularly relevant for small PbS QDs (with diameter of ≤4 nm) leading to absorption coefficients that largely differ from bulk values (up to ∼40% less). As a result of the broadband quantum confinement and of the high surface-to-volume ratio peculiar of nanocrystals, suitable surface chemical modification of PbS QDs is exploited to achieve a marked, size-dependent enhancement of the absorption coefficients compared to bulk values (up to ∼250%). We provide empirical relations to determine the absorption coefficients at 400 nm of as-synthesized and ligand-exchanged PbS QDs, accounting for the broadband quantum confinement and suggesting a heuristic approach to qualitatively predict the ligand effects on the optical absorption properties of PbS QDs. Our findings go beyond formalisms derived from Maxwell Garnett effective medium theory to describe QD optical properties and permit to spectrophotometrically calculate the concentration of PbS QD solutions avoiding underestimation due to deviations from the bulk. In perspective, we envisage the use of extended π-conjugated ligands bearing electronically active substituents to enhance light-harvesting in QD solids and suggest the inadequacy of the representation of ligands at the QD surface as mere electric dipoles.

UV Light Detection from CdS Nanocrystal Sensitized Graphene Photodetectors at kHz Frequencies

We have fabricated UV-sensitive photodetectors based on colloidal CdS nanocrystals and graphene. The nanocrystals act as a sensitizer layer that improves light harvesting leading to high responsivity of the detector. Despite the slow relaxation of the photogenerated charges in the nanocrystal film, faster processes allowed to detect pulses up to a repetition rate of 2 kHz. We have performed time-resolved analysis of the processes occurring in our hybrid system and discuss possible photoinduced charge transfer mechanisms.

Photophysical, photochemical, and electrochemical properties of dendrimers with a dimethoxybenzil core

Three dendrimers consisting of a dimethoxybenzil core and branches that contain two (G0), four (G1), and eight (G2) naphthalene units at the periphery and zero (G0), two (G1), and six (G2) dimethoxybenzene units in the branches have been synthesized and their photophysical, photochemical, and electrochemical properties have been investigated. For comparison purposes, the properties of dimethoxybenzil (MB) and of a dendron containing four naphthalene and three dimethoxybenzene units (D2) have also been studied. The properties of the dendrimers in the ground state (absorption spectra and electrochemical behavior) are those expected for their noninteracting component units. The excited state properties, however, are substantially controlled by electronic interactions between the dimethoxybenzil core and the naphthalene units contained in the branches. In dichloromethane–chloroform 1 : 1 (v/v) solution at 298 K, energy transfer from the lowest excited state (S1) of the naphthalene units to the lower lying S1 excited state of the dimethoxybenzil core takes place with high efficiency. In a rigid matrix at 77 K, selective excitation of the dimethoxybenzil chromophore yields an emission band that exhibits a spectral evolution: in the millisecond time scale it shows a spectral profile very similar to the dimethoxybenzil phosphorescence, whereas in the second time scale it is very similar to the naphthalene-type phosphorescence. Energy transfer from the T1 excited state of the dimethoxybenzil core to the T1 excited state of the naphthalene units takes place at 77 K, but not at 298 K, because the T1 excited state of the dimethoxybenzil core moves to energy lower than that of the naphthalene chromophore. The photochemical results show that the dimethoxybenzil core maintains its intrinsic photoreactivity toward dioxygen, and that on increasing dendrimer generation a photoreaction between core and branches predominates.

Surface Traps in Colloidal Quantum Dots: A Combined Experimental and Theoretical Perspective

Surface traps are ubiquitous to nanoscopic semiconductor materials. Understanding their atomistic origin and manipulating them chemically have capital importance to design defect-free colloidal quantum dots and make a leap forward in the development of efficient optoelectronic devices. Recent advances in computing power established computational chemistry as a powerful tool to describe accurately complex chemical species and nowadays it became conceivable to model colloidal quantum dots with realistic sizes and shapes. In this Perspective, we combine the knowledge gathered in recent experimental findings with the computation of quantum dot electronic structures. We analyze three different systems: namely, CdSe, PbS, and CsPbI3 as benchmark semiconductor nanocrystals showing how different types of trap states can form at their surface. In addition, we suggest experimental healing of such traps according to their chemical origin and nanocrystal composition.