Felice Gesuele

Ultrafast Supercontinuum Spectroscopy of Carrier Multiplication and Biexcitonic Effects in Excited States of PbS Quantum Dots

We examine the population dynamics of multiple excitons in PbS quantum dots using spectrally resolved ultrafast supercontinuum transient absorption (SC-TA) measurements. We simultaneously probe the first three excitonic transitions. The transient spectra show the presence of bleaching of absorption for the 1S(h)-1S(e) transition, as well as transients associated with the 1P(h)-1P(e) transition. We examine signatures of carrier multiplication (multiple excitons arising from a single absorbed photon) from analysis of the bleaching features in the limit of low absorbed photon numbers (left angle bracket N(abs) right angle bracket ∼ 10(-2)) for pump photon energies from two to four times that of the band gap. The efficiency of multiple-exciton generation is discussed both in terms of the ratio between early- to long-time transient absorption signals and of a broadband global fit to the data. Analysis of the population dynamics shows that bleaching associated with biexciton population is red shifted with respect to the single exciton feature, which is in accordance with a positive binding energy for the biexciton.

Time-resolved energy transfer from single chloride-terminated nanocrystals to graphene

We examine the time-resolved resonance energy transfer of excitons from single n-butyl amine-bound, chloride-terminated nanocrystals to two-dimensional graphene through time-correlated single photon counting. The radiative biexponential lifetime kinetics and blinking statistics of the individual surface-modified nanocrystal elucidate the non-radiative decay channels. Blinking modification as well as a 4× reduction in spontaneous emission were observed with the short chloride and n-butylamine ligands, probing the energy transfer pathways for the development of graphene-nanocrystal nanophotonic devices.

Green synthesis of luminescent and defect-free bio-nanosheets of MoS2: interfacing two-dimensional crystals with hydrophobins

Solution processing and biofunctionalization of two-dimensional crystals are pivotal for their (biomedical) applications. Here we interface ultrathin layers of MoS2 with the surface active and self-assembling fungal proteins named Vmh2, which belong to the hydrophobin family. We produce few-layered biofunctionalized MoS2 (bio-MoS2) nanosheets via liquid phase exfoliation in a green solvent, and controlled centrifugation; a low-cost and eco-friendly process. The dispersions are investigated by electrophoretic mobility, atomic force microscopy (AFM), UV-Vis, Raman and photoluminescence (PL) spectroscopy. The nanosheets present a defect-free vibrational spectrum, tunable zeta-potential and their photoluminescence is preserved after non-covalent biofunctionalization making them well suited for various biomedical applications.

Real-space observation of spectral degeneracy breaking in a waveguide-coupled disk microresonator

We report on the real-space observation of resonant frequency splitting in a high-Q waveguide-coupled silicon-on-insulator microdisk resonator. Phase sensitive near-field analysis reveals the stationary nature of the two resonant states, and spectral investigations clearly show their orthogonality. These measurements emphasize the role of the coupling waveguide in this splitting phenomenon. The symmetry of the two stationary whispering gallery modes is clearly observed and is found to follow the axial symmetry of the waveguide-coupled microdisk as it has been reported by earlier theoretical predictions.

Femtosecond UV-laser pulses to unveil protein–protein interactions in living cells

A hallmark to decipher bioprocesses is to characterize protein–protein interactions in living cells. To do this, the development of innovative methodologies, which do not alter proteins and their natural environment, is particularly needed. Here, we report a method (LUCK, Laser UV Cross-linKing) to in vivo cross-link proteins by UV-laser irradiation of living cells. Upon irradiation of HeLa cells under controlled conditions, cross-linked products of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were detected, whose yield was found to be a linear function of the total irradiation energy. We demonstrated that stable dimers of GAPDH were formed through intersubunit cross-linking, as also observed when the pure protein was irradiated by UV-laser in vitro. We proposed a defined patch of aromatic residues located at the enzyme subunit interface as the cross-linking sites involved in dimer formation. Hence, by this technique, UV-laser is able to photofix protein surfaces that come in direct contact. Due to the ultra-short time scale of UV-laser-induced cross-linking, this technique could be extended to weld even transient protein interactions in their native context.

Unconventional ratiometric-enhanced optical sensing of oxygen by mixed-phase TiO2

We show that mixed-phase titanium dioxide (TiO2) can be effectively employed as an unconventional, inorganic, dual-emitting, and ratiometric optical sensor of O2. Simultaneous availability of rutile and anatase TiO2 photoluminescence (PL) and their peculiar “anti-correlated” PL responses to O2 allow using their ratio as a measurement parameter associated with the O2 concentration, leading to an experimental responsivity being by construction larger than the one obtainable for single-phase PL detection. A proof of this concept is given, showing a two-fold enhancement of the optical responsivity provided by the ratiometric approach. Besides the peculiar ratiometric-enhanced responsivity, other characteristics of mixed phase TiO2 can be envisaged as favorable for O2 optical probing, namely (a) low production costs, (b) absence of heterogeneous components, and (c) self-supporting properties. These characteristics encourage experimenting with its use for applications requiring high indicator quantities at a co...

Giant O2-Induced Photoluminescence Modulation in Hierarchical Titanium Dioxide Nanostructures

We demonstrate exceptionally large modulation of PL intensity in hierarchical titanium dioxide (TiO2) nanostructures exposed to molecular oxygen (O2). Optical responsivities up to about 1100% at 20% O2 concentrations are observed in hyperbranched anatase-phase hierarchical structures, outperforming those obtainable by commercial TiO2 nanopowders (up to a factor of ∼7 for response to synthetic air) and significantly improving the ones typically reported in PL-based opto-chemical gas sensing using MOXs. The improved PL response is discussed in terms of the specific morphology of hierarchical structures, characterized by simultaneous presence of small nanoparticles, large surface areas, and large voids. These characteristics guarantee an optimal interplay between photogenerated charges, PL-active centers, and adsorbed gas molecules. The results highlight the potentialities offered by hierarchical structures based on TiO2 or other MOXs and open interesting scenarios toward the development of all-optical and/or hybrid (opto/electrical) chemical sensors with improved sensitivity.

Temporal and spectral characterization of femtosecond deep-UV chirped pulses

In contrast to the case of pulses in the infrared (IR) and visible range, the temporal characterization of deep-UV femtosecond pulses in combination with their spectral features is still a challenge, essentially due to the lack of suitable nonlinear crystals for second harmonic autocorrelation. Here we report on the characterization of 260 nm, nearly 200 fs pulses, based on two photon absorption in fused silica. 260 nm pulses are obtained as the fourth harmonic component of a near-IR fundamental which is frequency up-converted into a double beta barium borate-based harmonic generator stage. By comparing the obtained pulse duration with its Fourier limit, estimated by measuring pulse spectra, a consistent pulse chirp is retrieved. This chirp is mostly attributed to the considerable group-velocity dispersion occurring in the last doubling stage which converts the green into UV radiation. Additionally, the spectral width of the probe pulse through the fused silica window turns out to be modulated as a function of the time delay between pump and probe in the two-photon absorption setup. The observed modulation is attributed to the interplay between spectrally selective absorption, due to the chirp of the pulses, and moderate self-phase modulation just occurring at the top of the temporal autocorrelation between pump and probe.

Carbon Nanotube‐Quantum Dot Nanohybrids: Coupling with Single‐Particle Control in Aqueous Solution

A strategy is reported for the controlled assembly of organic-inorganic heterostructures consisting of individual single-walled carbon nanotubes (SWCNTs) selectively coupled to single semiconductor quantum dots (QDs). The assembly in aqueous solution was controlled towards the formation of monofunctionalized SWCNT-QD structures. Photoluminescence studies in solution, and on surfaces at the single nanohybrid level, showed evidence of electronic coupling between the two nanostructures. The ability to covalently couple heterostructures with single particle control is crucial for the design of novel QD-based optoelectronic and light-energy conversion devices.

Tuning the Coupling in Single-Molecule Heterostructures: DNA-Programmed and Reconfigurable Carbon Nanotube-Based Nanohybrids

Abstract Herein a strategy is presented for the assembly of both static and stimuli‐responsive single‐molecule heterostructures, where the distance and electronic coupling between an individual functional nanomoiety and a carbon nanostructure are tuned via the use of DNA linkers. As proof of concept, the formation of 1:1 nanohybrids is controlled, where single quantum dots (QDs) are tethered to the ends of individual carbon nanotubes (CNTs) in solution with DNA interconnects of different lengths. Photoluminescence investigations—both in solution and at the single‐hybrid level—demonstrate the electronic coupling between the two nanostructures; notably this is observed to progressively scale, with charge transfer becoming the dominant process as the linkers length is reduced. Additionally, stimuli‐responsive CNT‐QD nanohybrids are assembled, where the distance and hence the electronic coupling between an individual CNT and a single QD are dynamically modulated via the addition and removal of potassium (K+) cations; the system is further found to be sensitive to K+ concentrations from 1 pM to 25 × 10−3 m. The level of control demonstrated here in modulating the electronic coupling of reconfigurable single‐molecule heterostructures, comprising an individual functional nanomoiety and a carbon nanoelectrode, is of importance for the development of tunable molecular optoelectronic systems and devices.