Andrea Lamberti

A Chemometric Approach for the Sensitization Procedure of ZnO Flowerlike Microstructures for Dye-Sensitized Solar Cells

In this paper, a methodology for the streamlining of the sensitization procedure of flowerlike ZnO nanostructures for dye-sensitized solar cells (DSCs) is reported. The sensitization of ZnO surface with ruthenium-based complexes is a particularly critical process, since one has to minimize the dissolution of surface Zn atoms by the protons released from the dye molecules, leading to the formation of Zn(2+)/dye complexes. The fine-tuning of the experimental parameters, such as the dye loading time, the dye concentration, and the pH of the sensitizing solution, performed through a multivariate optimization by means of a chemometric approach, is here reported. The dye loading procedure was optimized using ZnO microparticles with nanostructured protrusions, synthesized by a simple and low-cost hydrothermal process. Mild reaction conditions were used, and wurtzite-like crystalline structure with a relatively high surface area was obtained once the reaction process was completed. After dispersion of ZnO flowerlike particles in an acetic acid-based solution, a 14 μm-thick ZnO layer acting as DSC photoanode was fabricated. The optimized sensitization procedure allowed minimizing the instability of ZnO surface in contact with acidic dyes, avoiding the formation of molecular agglomerates unable to inject electrons in the ZnO conduction band, achieving good results in the photoconversion efficiency. Moreover, the photoharvesting properties were further enhanced by adding N-methylbenzimidazole into the iodine-based liquid electrolyte. Such an additive was proposed here for the first time in combination with a ZnO photoelectrode, helping to reduce an undesired recombination between the photoinjected electrons and the oxidized redox mediator.

In situ MoS2 Decoration of Laser-Induced Graphene as Flexible Supercapacitor Electrodes

Herein, we are reporting a rapid one-pot synthesis of MoS2-decorated laser-induced graphene (MoS2-LIG) by direct writing of polyimide foils. By covering the polymer surface with a layer of MoS2 dispersion before processing, it is possible to obtain an in situ decoration of a porous graphene network during laser writing. The resulting material is a three-dimensional arrangement of agglomerated and wrinkled graphene flakes decorated by MoS2 nanosheets with good electrical properties and high surface area, suitable to be employed as electrodes for supercapacitors, enabling both electric double-layer and pseudo-capacitance behaviors. A deep investigation of the material properties has been performed to understand the chemical and physical characteristics of the hybrid MoS2-graphene-like material. Symmetric supercapacitors have been assembled in planar configuration exploiting the polymeric electrolyte; the resulting performances of the here-proposed material allow the prediction of the enormous potentialities of these flexible energy-storage devices for industrial-scale production.

PDMS membranes with tunable gas permeability for microfluidic applications

The air permeability of PDMS membranes is easily tuned by varying their composition. By varying the mixing ratio between oligomers and curing agent it is possible to strongly influence the chemical and mechanical properties of the elastomer resulting in a huge increase in the permeation of gas molecules across the membrane.

Ultrafast room-temperature crystallization of TiO2 nanotubes exploiting water-vapor treatment.

In this manuscript a near-room temperature crystallization process of anodic nanotubes from amorphous TiO2 to anatase phase with a fast 30 minutes treatment is reported for the first time. This method involves the exposure of as-grown TiO2 nanotubes to water vapor flow in ambient atmosphere. The water vapor-crystallized samples are deeply investigated in order to gain a whole understanding of their structural, physical and chemical properties. The photocatalytic activity of the converted material is tested by dye degradation experiment and the obtained performance confirms the highly promising properties of this low-temperature processed material.

Ultrasensitive Ag-coated TiO2 nanotube arrays for flexible SERS-based optofluidic devices

In this study, a novel SERS sensor has been developed for repeatable detection of organic molecules and biological assays. Vertically oriented titania nanotube (TiO2 NT) arrays were grown by ultra-fast anodic oxidation of flexible titanium foils and then decorated with Ag nanoparticles (NPs) through d.c. sputtering deposition at room temperature. A parametric study was carried out taking into account the effect of sputtering parameters on the Ag NP arrangements on the NT surface. The structure morphology was investigated by means of scanning and transmission electron microscopy, evidencing the formation of hexagonal close-packed TiO2 NTs coated with Ag nanoparticles showing tunable diameter and distribution. The substrates were employed in a SERS optofluidic device, consisting of a polydimethylsiloxane cover irreversibly sealed to the silver-coated TiO2 NTs, able to detect Rhodamine molecules in ethanol over a wide range of concentrations down to 10−14 M, taking advantage of both electromagnetic and chemical enhancements. In order to evaluate the performances of the SERS substrates in terms of biosensing, an optimized protocol for the immobilization of oligonucleotide probes on the metal-dielectric surfaces was developed for verifying the hybridization events.

Metal–elastomer nanostructures for tunable SERS and easy microfluidic integration

Stretchable plasmonic nanostructures constituted by Ag nanoparticles on flexible elastomeric matrices are synthesized and used as surface-enhanced Raman scattering (SERS) substrates. The structure consists of silver particles deposited by DC sputtering on polydimethylsiloxane (PDMS). The optical transmittance spectra show marked dips related to plasmonic inter-particle short-range interactions. The substrates show noticeable Raman enhancement allowing detection of R6G at very low concentration. Under mechanically controlled stretching, the interparticle gap sizes change yielding a reversible spectral shift of the plasmonic resonance. Experimental results are validated by 3D modeling. When such a resonance matches the Raman excitation line, pronounced enhancements can be achieved, optimizing the SERS regime. Taking advantage of the PDMS matrices, these tunable SERS-active substrates are integrated in microfluidic circuitry fruitfully exploitable for on-chip label-free detection.

Unveiling the controversial mechanism of reversible Na storage in TiO2 nanotube arrays: Amorphous versus anatase TiO2

Due to their inherent safety, low cost, and structural stability, TiO2 nanostructures represent a suitable choice as anode materials in sodium-ion batteries. In the recent years, various hypotheses have been proposed regarding the actual mechanism of the reversible insertion of sodium ions in the TiO2 structure, and previous reports are often controversial in this respect. Interestingly, when tested as binder- and conducting additive-free electrodes in laboratory-scale sodium cells, amorphous and crystalline (anatase) TiO2 nanotubular arrays obtained by simple anodic oxidation exhibit peculiar and intrinsically different electrochemical responses. In particular, after the initial electrochemical activation, anatase TiO2 shows excellent rate capability and very stable long-term cycling performance with larger specific capacities, and thus a clearly superior response compared with the amorphous counterpart. To obtain deeper insight, the present materials are thoroughly characterized by scanning electron microscopy and ex situ X-ray diffraction, and the insertion of sodium ions in the TiO2 bulk phases is systematically modeled by density functional theory calculations. The present results may contribute to the development of more systematic screening approaches to identify suitable active materials for highly efficient sodium-based energy storage systems.

Magnetoelastic Clock System for Nanomagnet Logic

In recent years, magnetic-based technologies, like nanomagnet logic (NML), are gaining increasing interest as possible substitutes of CMOS transistors. The possibility to mix logic and memory in the same device, coupled with a potential low power consumption, opens up completely new ways of developing circuits. The major issue of this technology is the necessity to use an external magnetic field as clock signal to drive the information through the circuit. The power losses due to the magnetic field generation potentially wipe out any advantages of NML. To solve this problem, new clock mechanisms were developed, based on spin transfer torque current and on voltage-controlled multiferroic structures that use magnetoelastic properties of magnetic materials, i.e., exploiting the possibility of influencing magnetization dynamics by means of the elastic tensor. In particular, the latter shows an extremely low power consumption. In this paper, we propose an innovative voltage-controlled magnetoelastic clock system aware of the technological constraints risen by modern fabrication processes. We show how circuits can be fabricated taking into account technological limitations, and we evaluate the performance of the proposed system. Results show that the proposed solution promises remarkable improvements over other NML approaches, even though state-of-the-art ideal multiferroic logic has in theory better performance. Moreover, since the proposed approach is technology-friendly, it gives a substantial contribution toward the fabrication of a full magnetic circuit and represents an optimal tradeoff between performance and feasibility.

Interfacial Effects in Solid-Liquid Electrolytes for Improved Stability and Performance of Dye-Sensitized Solar Cells

With the purpose of achieving stable dye-sensitized solar cells (DSSCs) with high efficiency, a new type of soft matter electrolyte is tested in which specific amounts of nanosized silica particles are finely dispersed in short-chained polyethylene glycol dimethylether encompassing an iodide/triiodide redox mediator. This results in a solid-liquid composite having synergistic electrical and favorable mechanical properties. The combination of interfacial effects and particle network formation promotes enhanced ion transport, which directly impacts the short-circuit photocurrent density. Thorough analysis reveals that this newly elaborated class of electrolytes is able to improve, at the same time, the thermal and long-term stability of DSSCs, as well as power conversion efficiency under standard and lower irradiation intensities. Lab-scale devices with champion efficiency exceeding 11% under attenuated sunlight (20 mW cm-2, with a compact TiO2 blocking layer) are obtained, along with impressively stable performance under both thermal stress and light soaking in an indoor environment (>96% performance retention after 2500 h of accelerated aging under full sun alternated with thermal ramps), matching the durability criteria applied to silicon solar cells for outdoor applications. The new findings might foster widespread practical application of DSSCs.

Surface label-free sensing by means of a fluorescent multilayered photonic structure

A fluorescent dielectric multilayer is exploited for label-free sensing in aqueous micro-environment. Fluorescence is laser-excited and collected through prism-coupling to a surface electromagnetic mode, also known as Bloch surface waves (BSW) localized at the interface between the multilayer and the outer aqueous medium. By detecting the spectral changes of the BSW-coupled light emission due to an external perturbation of the refractive index (Δn), a sensitivity of ∼2500 nm/RIU and a limit of detection down to Δn ∼ 3 × 10−6 are obtained.