[Show abstract][Hide abstract] ABSTRACT: Semiconducting nanocrystals optically active in the infrared region of the electromagnetic spectrum enable exciting avenues in fundamental research and novel applications compatible with the infrared transparency windows of biosystems such as chemical and biological optical sensing, including nanoscale thermometry. In this context, quantum dots (QDs) with double color emission may represent ultra-accurate and self-calibrating nanosystems. We present the synthesis of giant core/shell/shell asymmetric QDs having a PbS/CdS Zincblende (Zb)/CdS Wurtzite (Wz) structure with double color emission close to the near-infrared (NIR)region. We show that the double emission depends on the excitation condition and analyze the electron-hole distribution responsible of the independent and simultaneous radiative exciton recombination in the PbS core and in the CdS Wz shell, respectively. These results highlight the importance of the driving force leading to preferential crystal growth in asymmetric QDs, and provide a pathway for a rational control of the synthesis of double color emitting giant QDs, leading to the effective exploitation of visible/NIR transparency windows.
[Show abstract][Hide abstract] ABSTRACT: Quantum dots (QDs) represent one of the most promising materials for third generation solar cells, due to their potential to boost the photoconversion efficiency beyond the Shockley-Queisser limit. Composite nanocrystals can challenge the current scenario by combining broad spectral response and tailored energy levels to favor charge extraction, and reduce energy and charge recombination. We synthesized PbS/CdS QDs with different compositions at the surface of TiO2 nanoparticles assembled in a mesoporous film. The ultrafast photoinduced dynamics and the charge injection processes were investigated by pump-probe spectroscopy. We demonstrated good injection of photogenerated electrons from QDs to TiO2 in the PbS/CdS blend and used the QDs to fabricate solar cells. The fine tuning of chemical composition and size of lead and cadmium chalcogenide QDs led to highly efficient PV devices (3% maximum photoconversion efficiency). This combined study paves the way to the full exploitation of QDs in next generation PV devices.
No preview · Article · Jun 2015 · Journal of Physical Chemistry Letters
[Show abstract][Hide abstract] ABSTRACT: We study by femtosecond pump−probe microscopy the
transient plasmonic response of individual gold nanoantennas fabricated
by electron-beam lithography on a glass substrate. By exploiting the
capability of the fabrication technique to control geometrical parameters
at the nanoscale, we tuned the plasmonic resonance in a broad
wavelength range, from the visible to the infrared. Numerical simulations
based on a three-temperature model (3TM) for the electrons and lattice
dynamics, combined with full-wave numerical analysis and semiclassical
theory of optical transitions in the solid state, are compared with the
measurements on a single gold nanoantenna probed at different wavelengths. The agreement between the experiment and theprediction of the 3TM turns out to be comparable to that achievable with the more sophisticated Boltzmann equation formalism.
We also investigate the influence of the plasmon detuning with respect to the pump and probe wavelengths on the nonlinearoptical response using different nanoantennas. Quantitative comparison of the experimental data with the theoretical model also
provides a disentanglement of the different contributions to the optical nonlinearity of gold giving rise to the complex featuresobserved in the transient optical response. Our study provides a complete analysis of the physical mechanisms dominating the
nonlinear plasmon dynamics of an individual nano-object taking place on a few ps time scale
[Show abstract][Hide abstract] ABSTRACT: We report on the laser action in a microcavity where both the dielectric mirrors and the active material have a macromolecular nature, resulting in a full plastic laser device. Distributed Bragg reflectors (DBRs) are prepared by spin-coating of polyvinylcarbazole and cellulose acetate orthogonal solutions and the active layer consists of a highly fluorescent conjugated polymer poly(9,9-dioctylfluorenyl-2,7-diyl-co-1,4-benzo-(2,1'-3)-thiadiazole) (F8BT) spun film. A quality factor in the range 80–180 is achieved and the cavity mode is carefully tuned on the peak of the F8BT amplified spontaneous emission spectrum. Under pulsed optical pumping, laser emission is obtained with a surprisingly low lasing threshold (<20 μJ cm−2) for a full plastic DBR optical cavity. This result opens a simple and cheap way to obtain a new class of polymer lasers.
Full-text · Article · Mar 2014 · Laser Physics Letters
[Show abstract][Hide abstract] ABSTRACT: We report on the experimental investigation and theoretical modeling of the ultrafast nonlinear optical response exhibited by Cu2–xSe nanoparticles in a broad range of wavelengths, from the red to the near-infrared, under excitation with intense femtosecond-laser pulses. The ultrafast dynamics of the localized plasmon resonance of the system is mapped by pump–probe differential transmission measurements. Deviations from theoretical predictions based on standard two-temperature model (TTM) are revealed in the subpicosecond time-scale where the nonlinear phenomenon is more pronounced. The key-role played by nonthermalized carriers is investigated in detail on the basis of an extended TTM.
No preview · Article · Sep 2013 · Journal of Physical Chemistry Letters
[Show abstract][Hide abstract] ABSTRACT: Heavily-doped semiconductor nanocrystals characterized by a tunable plasmonic
band have been gaining increasing attention recently. Herein, we introduce this
type of materials focusing on their structural and photo physical properties.
Beside their continuous-wave plasmonic response, depicted both theoretically
and experimentally, we also review recent results on their transient, ultrafast
response. This was successfully interpreted by adapting models of the ultrafast
response of gold nanoparticles.
Full-text · Article · Jun 2013 · Physics of Condensed Matter
[Show abstract][Hide abstract] ABSTRACT: Here we show an innovative, simple and reliable method to fabricate micro-lasers by self-assembly of rod-shaped nanocrystals. We use dot/rod core/shell CdSe/CdS nanorods to form optical micro-resonators by exploiting their self-organization into well-defined coffee stain rings. The fabrication process merely consists of capillary jet deposition of a nanorod solution onto a glass substrate, and is scalable, economic, and highly reproducible. Upon optical pumping of the micro-resonators we obtain laser emission in the red or in the blue-green spectral region, demonstrating lasing both from core and shell transitions, with low pumping thresholds. Modeling by full-wave numerical simulations according to generalized (i. e. scattering) formulation of laser theory demonstrates lasing from complex modes of the self-assembled cavity.
[Show abstract][Hide abstract] ABSTRACT: CdSe/CdS dot/rods nanocrystals show interesting physical properties related to the band-alignment at the hetero-interface, which controls the band-edge electron delocalization over the rods. Here the differential transmission spectra of CdSe/CdS nanorod samples with different core sizes have been measured using excitation resonant to the core transition. The photo bleaching ratio between dot and rod transitions increases with the dot size, indicating a trend towards electron localization. This trend has been further quantified by performing effective mass calculations in which the conduction band misalignment was varied in order to reproduce the observed bleaching feature ratio. The best agreement was found for negligible conduction band misalignment for small dots of around 2.3 nm in diameter, and about -0.1 eV misalignment was estimated for the larger dots, above 3.5 nm in diameter. This shows that the band misalignment might be dependent on the geometry of the system, and we argue that this might be related to different strain developed at the hetero-interface.
[Show abstract][Hide abstract] ABSTRACT: We present a study of the intermediate energy transitions in octapod CdSe/CdS nanocrystals accomplished by ultrafast pump probe spectroscopy (150 fs resolution) combined with effective mass calculations. The bleaching features revealed in the differential transmission spectrum indicate that intermediate transitions occur from higher-energy hole states confined in the core, to the few electron states mildly localized in the core by the weak geometrical confinement. The detection of bleaching features of the intermediate states at long time implies that electron-hole recombination is inhibited in these structures, meanwhile electrons are available for further transport along the nanostructures. This information indicates that such nanostructures could be promising for photovoltaic applications.
No preview · Article · Apr 2012 · Physics of Condensed Matter
[Show abstract][Hide abstract] ABSTRACT: An innovative, simple and reliable method to fabricate micro-lasers by self-assembly of rod-shaped nanocrys-tals is demonstrated. Dot/rod core/shell CdSe/CdS nanorods are used to form optical micro-resonators by exploiting their self-organization into well-defined coffee stain rings. The fabrication process merely consists of capillary jet deposition of a nanorod solution onto a glass substrate, and is scalable, eco-nomic, and highly reproducible. Upon optical pumping of the micro-resonators, laser emission in the red or in the blue-green spectral region is obtained, demonstrating lasing both from core and shell transitions, with low pumping thresholds. Modeling by full-wave numerical simulations according to generalized (i. e. scattering) formulation of laser theory demonstrates lasing from complex modes of the self-assembled cavity.
[Show abstract][Hide abstract] ABSTRACT: Electro-optical mapping of the charge density with sub-micrometer resolution can be obtained in a high mobility, top-gate n-channel polymer field-effect transistor by charge modulation microscopy. Local features on the 1 μm scale are unveiled and, using scanning transmission X-ray microscopy measurements, are attributed to structural variations within the polymeric film.
No preview · Article · Nov 2011 · Advanced Materials
[Show abstract][Hide abstract] ABSTRACT: The optical response of metallic nanostructures after intense excitation with femtosecond-laser pulses has recently attracted increasing attention: such response is dominated by ultrafast electron-phonon coupling and offers the possibility to achieve optical modulation with unprecedented terahertz bandwidth. In addition to noble metal nanoparticles, efforts have been made in recent years to synthesize heavily doped semiconductor nanocrystals so as to achieve a plasmonic behavior with spectrally tunable features. In this work, we studied the dynamics of the localized plasmon resonance exhibited by colloidal Cu(2-x)Se nanocrystals of 13 nm in diameter and with x around 0.15, upon excitation by ultrafast laser pulses via pump-probe experiments in the near-infrared, with ∼200 fs resolution time. The experimental results were interpreted according to the two-temperature model and revealed the existence of strong nonlinearities in the plasmonic absorption due to the much lower carrier density of Cu(2-x)Se compared to noble metals, which led to ultrafast control of the probe signal with modulation depth exceeding 40% in transmission.
[Show abstract][Hide abstract] ABSTRACT: Colloidal branched nanocrystals have been attracting increasing attention due to evidence of an interesting relationship between their complex shape and charge carrier dynamics. Herein, continuous wave photoinduced absorption (CW PIA) measurements of CdSe/CdS octapod-shaped nanocrystals are reported. CW PIA spectra show strong bleaching due to the one-dimensional (1D) CdS pod states (480 nm) and the zero-dimensional (0D) CdSe core states (690 nm). The agreement with previously reported ultrafast pump-probe experiments indicates that this strong bleaching signal may be assigned to state filling. Additional bleaching features at 520 and 560 nm are characterized by a longer lifetime and are thus ascribed to defect states, localized at the pod-core interface of the octapod, showing that some of the initially photogenerated carriers get quickly trapped into these long-lived defect states. However, we remark that a relevant part of electrons remain untrapped: this opens up the opportunity to exploit octapod shaped nanocrystals in photovoltaics applications, as electron acceptor materials, considering that several efficient hole extracting materials are already available for the realization of a composite bulk heterojunction.
No preview · Article · Sep 2011 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: We studied the carrier dynamics in colloidal octapod-shaped cadmium selenide/cadmium sulfide (CdSe/CdS) nanocrystals in the solution phase via pump-probe optical techniques with subpicosecond resolution. We could resolve bleaching from two different types of electronic states having distinct dynamics and assigned them to states delocalized in the pods and mildly localized in the core based on the good agreement of energies found with effective mass modeling. Contrary to other CdSe/CdS core/shell nanocrystals, such a mild localization has geometrical origins as the best agreement was found for negligible conduction band offset. Moreover, even though the large surface of the CdSe/CdS heterointerface results into a weak signature of electron trapping in the bleaching spectrum, we found that a relevant fraction of electrons do remain delocalized in pod states for long times and are thus available for diffusion in photovoltaic applications where the highly branched geometry is expected to advantageously yield to effective percolation in dense assemblies.
No preview · Article · May 2011 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: We studied the optical properties of core-shell CdSe/CdS nanorods with various lengths and core diameters that were fabricated by wet chemical synthesis using the seeded growth method. We investigated the optical emission from thin films consisting of dense nanorod arrays, where we observed amplified spontaneous emission from states related either to the CdSe core or to the CdS shell depending on the nanorod's length. The optical gain of the nanorods was studied by transient absorption experiments and we found optical gain for the core and shell states of short rods, whereas for long rods, the optical gain of the core was quenched by defect states and we observed optical gain solely from the states of the shell material. (C) 2011 American Institute of Physics. [doi:10.1063/1.3549298]
Full-text · Article · Feb 2011 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Two-photon pumped lasing of a poly(phenylenevinylene)-titania-silica (PPV-TiO2-SiO2) nanocomposite comprised of PPV synthesized within the void spaces of a multilayer nanoparticle TiO2 and SiO2 1D photonic crystal is reported for the first time. With this distributed feedback (DFB) device, we have discovered the surprising result that two photon lasing requires only a factor of 2 higher intensity than one photon lasing thereby allowing laser excitation densities far below those that cause material degradation. The lower excitation density makes possible laser operation in the high-repetition rate regime and the use of compact and cheap NIR pump sources. All this makes our device rather interesting for optical telecommunication applications.
No preview · Article · Nov 2010 · Chemistry of Materials
[Show abstract][Hide abstract] ABSTRACT: We studied carrier dynamics in semiconductor nanocrystals consisting of a small CdSe dot embedded in an elongated, rod-shaped CdS shell, using the ultrafast pump-probe technique. We found clear evidence of a substantial suppression of the Auger nonradiative recombination in the biexciton regime. Moreover, a simple model of the dynamics in which biexcitons show no Auger recombination, and only holes are localized in the dot, fits well the differential transmission observed at all pump densities. The long biexciton lifetime results into an observed long-living gain having a peak that is red shifted with respect to the lowest energy absorption peak. We argue that the origin of the large relative gain observed at large fillings is related to the peculiar structure of the electronic levels, and in particular, to delocalization of electrons in the rod.
[Show abstract][Hide abstract] ABSTRACT: Colloidal semiconductor quantum rods have demonstrated many advantageous properties as light emitters such as high quantum yield, tunable emission wavelength, and polarized emission. This makes them an interesting optical gain material for laser applications. We report room-temperature gain lifetimes in core/shell CdSe/CdS quantum rods exceeding 300 ps, and show that the long gain lifetimes result from the significant reduction of Auger recombination in our quantum rods, even though the electrons are delocalized over the rod volume. We also fabricate devices by deposition of small droplets of quantum rod solution onto flat substrates. The evaporation dynamics of the droplets are governed by the coffee stain effect which leads to the formation of well defined micron-size stripes. These stripes consist of densely packed, laterally aligned quantum rods and provide optical feedback originating from the abrupt changes of refractive index at the stripe borders. We exploit the optical gain and the coffee stain mediated self-assembly and show that we can fabricate novel microlasers solely by deposition of droplets of quantum rod solutions on flat substrates.
[Show abstract][Hide abstract] ABSTRACT: We studied the delocalization of electron wave function in asymmetric CdSe/CdS nanocrystals, consisting of a spherical CdSe dot embedded in an elongated US shell, by means of a pump-probe technique. By comparing the transient spectra obtained upon pumping the band edge transition of the CdSe in CdSe/CdS heterostructure and in a bare CdSe dot, we observed the delocalization of electron wave function at the CdSe/CdS interface. (C) 2009 Elsevier Ltd. All rights reserved.
No preview · Article · Jan 2010 · Superlattices and Microstructures
[Show abstract][Hide abstract] ABSTRACT: The complex optical polarizability of a nano-oscillator determines the way it couples to light and to other nano-objects in its environment. Hence, its experimental evaluation at the single-particle level represents a crucial task in nano-optics. In this work we demonstrate that both phase and amplitude of a nanoresonator polarizability, which are embedded in its near-field response, can be decoupled by combining near-field and confocal far-field extinction imagings. The interpretation of our measurements based on a simple analytical model is further confirmed by finite-difference time-domain calculations.
Full-text · Article · Oct 2009 · Physical Review B