In-Operando Optical Spectroscopy of Field-Effect-Gated Al-Doped ZnO.

Transparent conductive oxides (TCO) have the unique characteristics of combining optical transparency with high electrical conductivity; such a property makes them uniquely alluring for applications in visible and infrared photonics. One of their most interesting features is the large sensitivity of their optical response to the doping level. We performed the active electrical manipulation of the dielectric properties of aluminum-doped ZnO (AZO), a TCO-based on Earth-abundant elements. We actively tuned the optical and electric performances of AZO films by means of an applied voltage in a parallel-plate capacitor configuration, with SrTiO3 as the dielectric, and monitored the effect of charge injection/depletion by means of in-operando spectroscopic ellipsometry. Calculations of the optical response of the gated system allowed us to extract the spatially resolved variations in the dielectric function of the TCO and infer the injected/depleted charge profile at the interface.

Laser-Induced Morphological and Structural Changes of Cesium Lead Bromide Nanocrystals.

Metal halide perovskite nanocrystals, an emerging class of materials for advanced photonic and optoelectronic applications, are mainly fabricated with colloidal chemistry routes. On the quest for new properties according to application needs, new perovskite systems of various morphologies and levels of doping and alloying have been developed, often also involving post-synthesis reactions. Recently, laser irradiation in liquids has been utilized as a fast method to synthesize or transform materials and interesting laser-induced transformations on nanocrystals were induced. These studies in general have been limited to small nanocrystals (~15 nm). In the case of halide perovskites, fragmentation or anion exchange have been observed in such laser-based processes, but no crystal structure transformations were actually observed or deliberately studied. Nanocrystals are more sensitive to light exposure compared to the corresponding bulk crystals. Additional factors, such as size, morphology, the presence of impurities, and others, can intricately affect the photon absorption and heat dissipation in nanocrystal suspensions during laser irradiation. All these factors can play an important role in the final morphologies and in the time required for these transformations to unfold. In the present work, we have employed a 513 nm femtosecond (fs) laser to induce different transformations in large nanocrystals, in which two phases coexist in the same particle (Cs4PbBr6/CsPbBr3 nanohexagons of ~100 nm), dispersed in dichlorobenzene. These transformations include: (i) the exfoliation of the primary nanohexagons and partial anion exchange; (ii) fragmentation in smaller nanocubes and partial anion exchange; (iii) side-by-side-oriented attachment, fusion, and formation of nanoplatelets and complete anion exchange; (iv) side-by-side attachment, fusion, and formation of nanosheets. Partial or complete Br-Cl anion exchange in the above transformations was triggered by the partial degradation of dichlorobenzene. In addition to the detailed analysis of the various nanocrystal morphologies observed in the various transformations, the structure-photoluminescence relationships for the different samples were analyzed and discussed.

Doping-Dependent Optical Response of a Hybrid Transparent Conductive Oxide/Plasmonic Medium.

Understanding the interaction between plasmonic nanoparticles and transparent conductive oxides is instrumental to the development of next-generation photovoltaic, optoelectronic, and energy-efficient solid-state lighting devices. We investigated the optical response of hybrid media composed of gold nanoparticles deposited on aluminum-doped zinc oxide thin films with varying doping concentration by spectroscopic ellipsometry. The dielectric functions of bare AZO were addressed first, revealing doping-induced effects such as the band gap shift and the appearance of free carriers. In the hybrid media, a blue-shift of the localized surface plasmon resonance of Au NPs as a function of increasing Al doping of the substrate was observed, ascribed to the occurrence of a charge transfer between the two materials and the doping-dependent variation of the polarizability of the substrate.

Effective medium optical modelling of indium tin oxide nanocrystal films.

Doped semiconductor nanocrystal-based thin films are widely used for many applications, such as screens, electrochromic windows, light emitting diodes, and solar cells. Herein, we have employed spectroscopic ellipsometry to measure and model the complex dielectric response of indium tin oxide films fabricated by nanocrystal deposition and sintering. The films could be modelled as Bruggemann effective media, allowing estimation of the nanoscale interstitial porosity of the structure. The effective dielectric constants show the possibility of tuning the plasma frequency and the epsilon-near zero condition of the film.

Quantitative Ultrafast Electron-Temperature Dynamics in Photo-Excited Au Nanoparticles.

The femtosecond evolution of the electronic temperature of laser-excited gold nanoparticles is measured, by means of ultrafast time-resolved photoemission spectroscopy induced by extreme-ultraviolet radiation pulses. The temperature of the electron gas is deduced by recording and fitting high-resolution photo emission spectra around the Fermi edge of gold nanoparticles providing a direct, unambiguous picture of the ultrafast electron-gas dynamics. These results will be instrumental to the refinement of existing models of femtosecond processes in laterally-confined and bulk condensed-matter systems, and for understanding more deeply the role of hot electrons in technological applications.

Anion exchange in inorganic perovskite nanocrystal polymer composites.

We demonstrate a facile, low-cost and room-temperature method of anion exchange in cesium lead bromide nanocrystals (CsPbBr3 NCs), embedded into a polymer matrix. The anion exchange occurs upon exposure of the solid CsPbBr3 NCs/PDMS nanocomposite to a controlled anion precursor gas atmosphere. The rate and extent of the anion exchange reaction can be controlled via the variation of either the exposure time or the relative concentration of the anion precursor gas. Post-synthesis chemical transformation of perovskite nanocrystal-polymer composites is not readily achievable using conventional methods of anion exchange, which renders the gas-assisted strategy extremely useful. We envisage that this work will enable the development of solid-state perovskite NC optoelectronic devices.

Unveiling the Structure of MoS x Nanocrystals Produced upon Laser Fragmentation of MoS2 Platelets.

Transition-metal dichalcogenide MoS2 nanostructures have attracted tremendous attention due to their unique properties, which render them efficient nanoscale functional components for multiple applications ranging from sensors and biomedical probes to energy conversion and storage devices. However, despite the wide application range, the possibility to tune their size, shape, and composition is still a challenge. At the same time, the correlation of the structure with the optoelectronic properties is still unresolved. Here, we propose a new method to synthesize various morphologies of molybdenum sulfide nanocrystals, on the basis of ultrashort-pulsed laser fragmentation of MoS2 platelets. Depending on the irradiation conditions, multiple MoS x morphologies in the form of nanoribbons, nanospheres, and photoluminescent quantum dots are obtained. Besides the detailed structural analysis of the various crystals formed, the structure-property relation is investigated and discussed.

Advanced Photonic Processes for Photovoltaic and Energy Storage Systems.

Solar-energy harvesting through photovoltaic (PV) conversion is the most promising technology for long-term renewable energy production. At the same time, significant progress has been made in the development of energy-storage (ES) systems, which are essential components within the cycle of energy generation, transmission, and usage. Toward commercial applications, the enhancement of the performance and competitiveness of PV and ES systems requires the adoption of precise, but simple and low-cost manufacturing solutions, compatible with large-scale and high-throughput production lines. Photonic processes enable cost-efficient, noncontact, highly precise, and selective engineering of materials via photothermal, photochemical, or photophysical routes. Laser-based processes, in particular, provide access to a plethora of processing parameters that can be tuned with a remarkably high degree of precision to enable innovative processing routes that cannot be attained by conventional approaches. The focus here is on the application of advanced light-driven approaches for the fabrication, as well as the synthesis, of materials and components relevant to PV and ES systems. Besides presenting recent advances on recent achievements, the existing limitations are outlined and future possibilities and emerging prospects discussed.

Solution processed reduced graphene oxide electrodes for organic photovoltaics.

Since the isolation of free standing graphene in 2004, graphene research has experienced a phenomenal growth. Due to its exceptional electronic, optical and mechanical properties, graphene is believed to be the next wonder material for optoelectronics. The enhanced electrical conductivity, combined with its high transparency in the visible and near-infrared regions of the spectrum, enabled graphene to be an ideal low cost indium-tin oxide (ITO) substitute. Solution-processed reduced graphene oxide combines the unique optoelectrical properties of graphene with large area deposition and flexible substrates rendering it compatible with roll-to-roll manufacturing methods. This paper provides an overview of recent research progress in the application and consequent physical-chemical properties of solution-processed reduced graphene oxide-based films as transparent conductive electrodes (TCEs) in organic photovoltaic (OPV) cells. Reduced graphene oxide (rGO) can be effectively utilized as the TCE in flexible OPVs, where the brittle and expensive ITO is incompatible. The prospects and future research trends in graphene-based TCEs are also discussed.

Enhanced Stability of Aluminum Nanoparticle-Doped Organic Solar Cells.

Enhancement of the stability of bulk heterojunction (BHJ) organic photovoltaic (OPV) devices is reported by the addition of surfactant-free aluminum (Al) nanoparticles (NPs) into the photoactive layer. The universality of the effect is demonstrated for two different BHJ systems, namely, the well-studied poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) as well as the high efficient poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]:[6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC71BM). It is shown that the lifetime of the devices with Al NPs, operating under continuous one-sun illumination in ambient conditions, is more than three times longer compared to the reference devices. Using complementary analytical techniques for in situ studies, we have explored the underlying mechanisms behind the observed stability improvement in the case of the P3HT:PCBM system. In particular, laser-induced fluorescence (LIF), photoluminescence decay and Fourier transform infrared (FTIR) spectroscopy experiments were performed and complemented with device degradation electrical measurements. It is found that the embedded Al NPs act as performance stabilizers, giving rise to enhanced structural stability of the active blend. Furthermore, it is revealed that the observed improvement can also be ascribed to NP-mediated mitigation of the photo-oxidation effect. This study addresses a major issue in OPV devices, that is, photoinduced stability, indicating that the exploitation of Al NPs could be a successful approach toward fabricating OPVs exhibiting long-term operating lifetimes.

Assessment of in-depth degradation of artificially aged triterpenoid paint varnishes using nonlinear microscopy techniques.

The present work investigates the applicability of nonlinear imaging microscopy for the precise assessment of degradation of the outer protective layers of painted artworks as a function of depth due to aging. Two fresh and artificially aged triterpenoid varnishes, dammar and mastic, were tested. Nonlinear imaging techniques have been employed as a new diagnostic tool for determination of the exact thickness of the affected region due to artificial aging of the natural varnishes. The measured thicknesses differ from the calculated mean penetration depths of the samples. These nondestructive, high resolution modalities are valuable analytical tools for aging studies and they have the potential to provide unique in-depth information. Single photon laser induced fluorescence measurements and Raman spectroscopy were used for the integrated investigation and analysis of aging effects in varnishes.