Publications by authors named "Pasqualino Maddalena"

23 Publications

  • Page 1 of 1

Charge Carrier Processes and Optical Properties in TiO and TiO-Based Heterojunction Photocatalysts: A Review.

Materials (Basel) 2021 Mar 27;14(7). Epub 2021 Mar 27.

Department of Physics "E. Pancini", University of Naples "Federico II", Complesso Universitario di Monte S. Angelo, Via Cupa Cintia 21, 80126 Napoli, Italy.

Photocatalysis based technologies have a key role in addressing important challenges of the ecological transition, such as environment remediation and conversion of renewable energies. Photocatalysts can in fact be used in hydrogen (H) production (e.g., via water splitting or photo-reforming of organic substrates), CO reduction, pollution mitigation and water or air remediation via oxidation (photodegradation) of pollutants. Titanium dioxide (TiO) is a "benchmark" photocatalyst, thanks to many favorable characteristics. We here review the basic knowledge on the charge carrier processes that define the optical and photophysical properties of intrinsic TiO. We describe the main characteristics and advantages of TiO as photocatalyst, followed by a summary of historical facts about its application. Next, the dynamics of photogenerated electrons and holes is reviewed, including energy levels and trapping states, charge separation and charge recombination. A section on optical absorption and optical properties follows, including a discussion on TiO photoluminescence and on the effect of molecular oxygen (O) on radiative recombination. We next summarize the elementary photocatalytic processes in aqueous solution, including the photogeneration of reactive oxygen species (ROS) and the hydrogen evolution reaction. We pinpoint the TiO limitations and possible ways to overcome them by discussing some of the "hottest" research trends toward solar hydrogen production, which are classified in two categories: (1) approaches based on the use of engineered TiO without any cocatalysts. Discussed topics are highly-reduced "black TiO", grey and colored TiO, surface-engineered anatase nanocrystals; (2) strategies based on heterojunction photocatalysts, where TiO is electronically coupled with a different material acting as cocatalyst or as sensitizer. Examples discussed include TiO composites or heterostructures with metals (e.g., Pt-TiO, Au-TiO), with other metal oxides (e.g., CuO, NiO, etc.), direct Z-scheme heterojunctions with g-CN (graphitic carbon nitride) and dye-sensitized TiO.
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http://dx.doi.org/10.3390/ma14071645DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8036967PMC
March 2021

Time-gated luminescence imaging of positively charged poly-l-lysine-coated highly microporous silicon nanoparticles in living Hydra polyp.

J Biophotonics 2020 12 16;13(12):e202000272. Epub 2020 Sep 16.

Institute of Applied Sciences and Intelligent Systems, CNR, Naples, Italy.

The development of non-toxic fluorescent agents alternative to heavy metal-based semiconductor quantum dots represents a relevant topic in biomedical research and in particular in the bioimaging field. Herein, highly luminescent Si─H terminal microporous silicon nanoparticles with μs-lived photoemission are chemically modified with a two step process and successfully used as label-free probes for in vivo time-gated luminescence imaging. In this context, Hydra vulgaris is used as model organism for in vivo study and validity assessment. The application of time gating allows to pursue an effective sorting of the signals, getting rid of the most common sources of noise that are fast-decay tissue autofluorescence and excitation scattering within the tissue. Indeed, an enhancement by a factor ~ 20 in the image signal-to-noise ratio can be estimated.
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http://dx.doi.org/10.1002/jbio.202000272DOI Listing
December 2020

Structural and electronic properties of defective 2D transition metal dichalcogenide heterostructures.

J Comput Chem 2020 Aug 16;41(22):1946-1955. Epub 2020 Jun 16.

Department of Chemical Sciences, University of Naples Federico II, Naples, Italy.

We present a first-principles study on the structure-property relationships in MoS and WS monolayers and their vertically stacked hetero-bilayer, with and without Sulfur vacancies, in order to dissect the electronic features behind their photocatalytic water splitting capabilities. We also benchmark the accuracy of three different exchange-correlation density functionals for both minimum-energy geometries and electronic structure. The best compromise between computational cost and qualitative accuracy is achieved with the HSE06 density functional on top of Perdew-Burke-Ernzerhof minima, including dispersion with Grimme's D3 scheme. This computational approach predicts the presence of mid-gap states for defective monolayers, in accordance with the present literature. For the heterojunction, we find unexpected vacancy-position dependent electronic features: the location of the defects leads either to mid-gap trap states, detrimental for photocatalyst or to a modification of characteristic type II band alignment behavior, responsible for interlayer charge separation and low recombination rates.
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http://dx.doi.org/10.1002/jcc.26364DOI Listing
August 2020

Quantitative Study of Morphological Features of Stem Cells onto Photopatterned Azopolymer Films.

J Funct Biomater 2020 Feb 14;11(1). Epub 2020 Feb 14.

Institute of Polymers, Composites and Biomaterials, National Research Council of Italy, Mostra D'Oltremare, Pad.20, V.le J.F. Kennedy 54, 80125 Naples, Italy.

In the last decade, the use of photolithography for the fabrication of structured substrates with controlled morphological patterns that are able to interact with cells at micrometric and nanometric size scales is strongly growing. A promising simple and versatile microfabrication method is based on the physical mass transport induced by visible light in photosensitive azobenzene-containing polymers (or azopolymers). Such light-driven material transport produces a modulation of the surface of the azopolymer film, whose geometry is controlled by the intensity and the polarization distributions of the irradiated light. Herein, two anisotropic structured azopolymer films have been used as substrates to evaluate the effects of topological signals on the in vitro response of human mesenchymal stem cells (hMSCs). The light-induced substrate patterns consist of parallel microgrooves, which are produced in a spatially confined or over large-scale areas of the samples, respectively. The analysis of confocal optical images of the in vitro hMSC cells grown on the patterned films offered relevant information about cell morphology-i.e., nuclei deformation and actin filaments elongation-in relation to the geometry and the spatial extent of the structured area of substrates. The results, together with the possibility of simple, versatile, and cost-effective light-induced structuration of azopolymers, promise the successful use of these materials as anisotropic platforms to study the cell guidance mechanisms governing in vitro tissue formation.
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http://dx.doi.org/10.3390/jfb11010008DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7151610PMC
February 2020

Spontaneous wrinkle emergence in nascent eumelanin thin films.

Soft Matter 2019 Dec 29;15(45):9261-9270. Epub 2019 Oct 29.

INFN, Sezione di Napoli, Via Cintia 2, 80126 Napoli, Italy and Institute for Polymers, Composites and Biomaterials (IPCB), CNR, Via Campi Flegrei 34, 80078 Pozzuoli (Na), Italy. and National Interuniversity Consortium of Materials Science and Technology (INSTM), Via G. Giusti, 9, 50121 Florence, Italy.

Self-patterning processes originated by physical stimuli have been extensively documented in thin films, whereas spontaneous wrinkling phenomena due to chemical transformation processes are, to the best of our knowledge, unprecedented. Herein we report a case of spontaneous polymerization-driven surface nano-patterning (∼500 nm) that develops in smooth thin solid films of 5,6-dihydroxyindole (DHI), a major precursor of eumelanin polymers, over a time scale of 30 to 60 days in air at room temperature. The phenomenon can be observed only above a critical film thickness of ∼250 nm and it is affected by exposure to ammonia vapors causing acceleration of the oxidation process. The thickness-dependent onset of wrinkling can be attributed to non-homogeneous rates of oxidation through the film causing slow swelling/expansion of the inner layers followed by fast stiffening and cross-linking in the outer layer exposed to higher oxygen levels.
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http://dx.doi.org/10.1039/c9sm01885aDOI Listing
December 2019

Luminescent -Iridium(III) Complex Based on a Bis(6,7-dimethoxy-3,4-dihydroisoquinoline) Platform Featuring an Unusual cis Orientation of the CN Ligands: From a Theoretical Approach to a Deep Red LEEC Device.

ACS Omega 2019 Jan 25;4(1):2009-2018. Epub 2019 Jan 25.

Department of Chemical Sciences and Department of Physics "E. Pancini", University of Naples Federico II, via Cintia 4, I-80126 Napoli, Italy.

By pursuing the strategy of manipulating natural compounds to obtain functional materials, in this work, we report on the synthesis and characterization of a luminescent cationic iridium complex (-), designed starting from the catecholic neurotransmitter dopamine, exhibiting the unusual cis arrangement of the CN ligands. Through an integrated experimental and theoretical approach, it was possible to delineate the optoelectronic properties of -. In detail, (a) a series of absorption maxima in the range 300-400 nm was assigned to metal-to-ligand charge transfer and weak and broad absorption maxima at longer wavelengths (400-500 nm) were ascribable to spin-forbidden transitions with a mixed character; (b) there was an intense red phosphorescence with emission set in the range 580-710 nm; and (c) a highest occupied molecular orbital was mainly localized on the metal and the 2-phenylpiridine ligand and a lowest unoccupied molecular orbital was localized on the NN ligand, with a Δ set at 2.20 eV. This investigation allowed the design of light-emitting electrochemical cell (LEEC) devices endowed with good performance. The poor literature reporting on the use of -iridium(III) complexes in LEECs prompted us to investigate the role played by the selected cathode and the thickness of the emitting layer, as well as the doping effect exerted by ionic liquids on the performance of the devices. All the devices exhibited a deep red emission, in some cases, quite near the pure color (devices #1, #4, and #8), expanding the panorama of the iridium-based red-to-near-infrared LEEC devices. The characteristics of the devices, such as the brightness reaching values of 162 cd/m for device #7, suggested that the performances of - are comparable to those of trans isomers, opening new perspective toward designing a new set of luminescent materials for optoelectronic devices.
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http://dx.doi.org/10.1021/acsomega.8b02859DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6648618PMC
January 2019

Hexamethylenediamine-Mediated Polydopamine Film Deposition: Inhibition by Resorcinol as a Strategy for Mapping Quinone Targeting Mechanisms.

Front Chem 2019 5;7:407. Epub 2019 Jun 5.

Department of Chemical Sciences, University of Naples Federico II, Naples, Italy.

Hexamethylenediamine (HMDA) and other long chain aliphatic diamines can induce substrate-independent polymer film deposition from dopamine and several other catechols substrates at relatively low concentrations, however the mechanism of the diamine-promoted effect has remained little understood. Herein, we report data indicating that: (a) film deposition from 1 mM HMDA and dopamine is not affected by chemical oxidation with periodate but is markedly inhibited by resorcinol, which also prevents PDA film formation at 10 mM monomer concentration in the absence of HMDA; (b) N-acetylation of HMDA completely inhibits the effect on PDA film formation; (c) HMDA enables surface functionalization with 1 mM 5,6-dihydroxyindole (DHI) polymerization at pH 9.0 in a resorcinol-inhibitable manner. Structural investigation of the polymers produced from dopamine and DHI in the presence of HMDA using solid state C and N NMR and MALDI-MS suggested formation of covalent cross linked structures. It is concluded that HMDA enhances polydopamine adhesion by acting both on dopamine quinone and downstream, e.g., via covalent coupling with DHI. These results provide new insights into the mechanisms of PDA adhesion and disclose resorcinol as a new potent tool for targeting/mapping quinone intermediates and for controlling polymer growth.
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http://dx.doi.org/10.3389/fchem.2019.00407DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6560077PMC
June 2019

The Chemistry of Polydopamine Film Formation: The Amine-Quinone Interplay.

Biomimetics (Basel) 2018 Sep 13;3(3). Epub 2018 Sep 13.

Department of Chemical Sciences, University of Naples Federico II, Via Cinthia 4, I-80126 Naples, Italy.

Despite extensive investigations over the past decade, the chemical basis of the extraordinary underwater adhesion properties of polydopamine (PDA) has remained not entirely understood. The bulk of evidence points to PDA wet adhesion as a complex process based on film deposition, and growth in which primary amine groups, besides catechol moieties, play a central role. However, the detailed interplay of chemical interactions underlying the dynamics of film formation has not yet been elucidated. Herein, we report the results of a series of experiments showing that coating formation from dopamine at pH 9.0 in carbonate buffer: (a) Requires high dopamine concentrations (>1 mM); (b) is due to species produced in the early stages of dopamine autoxidation; (c) is accelerated by equimolar amounts of periodate causing fast conversion to the -quinone; and (d) is enhanced by the addition of hexamethylenediamine (HMDA) and other long chain aliphatic amines even at low dopamine concentrations (<1 mM). It is proposed that concentration-dependent PDA film formation reflects the competition between intermolecular amine-quinone condensation processes, leading to adhesive cross-linked oligomer structures, and the intramolecular cyclization route forming little adhesive 5,6-dihydroxyindole (DHI) units. Film growth would then be sustained by dopamine and other soluble species that can be adsorbed on the surface.
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http://dx.doi.org/10.3390/biomimetics3030026DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6352855PMC
September 2018

Computer-generated holograms for complex surface reliefs on azopolymer films.

Sci Rep 2019 May 1;9(1):6775. Epub 2019 May 1.

Center for Nanoscale Systems, Harvard University, 9 Oxford Street, Cambridge, Massachusetts, 02138, United States.

The light-driven superficial structuration observed on the surface of films of azobenzene-containing polymers follows the optical field distribution of the illuminating light pattern, i.e. the light polarization state and the intensity distribution. The ability to precisely manipulate the illuminating intensity pattern can hence provide a new level in the range of complex light-induced superficial textures accessible onto azopolymer film surfaces. In this respect, digital holography, based on the principles of the Computer-Generated Holograms (CGHs), and actually implemented by means of a versatile liquid crystal spatial light modulator, can represent a unique experimental tool in the field of the light-induced mass migration in azo-materials. In the present work, we demonstrate the possibility to precisely control the features and the quality of complex light patterns generated through CGHs in order to induce arbitrarily complex surface reliefs onto the surface of an azopolymer. The results shown here can potentially broaden the range of possible applications of photo-responsive azopolymer films in the fields of surface engineering, biology and photonics.
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http://dx.doi.org/10.1038/s41598-019-43256-wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6494893PMC
May 2019

Study of Anchoring Groups for CuGaO Delafossite-Based p-Type Dye Sensitized Solar Cells.

Front Chem 2019 29;7:158. Epub 2019 Mar 29.

Department of Chemical Sciences, University of Naples "Federico II", Comp. Univ. Monte Sant'Angelo, Naples, Italy.

Here we report the first theoretical characterization of the interface between the CuGaO delafossite oxide and the carboxylic (-COOH) and phosphonic acid (-POH) anchoring groups. The promising use of delafossites as effective alternative to nickel oxide in p-type DSSC is still limited by practical difficulties in sensitizing the delafossite surface. Thus, this work provides atomistic insights on the structure and energetics of all the possible interactions between the anchoring functional groups and the CuGaO surface species, including the effects of the Mg doping and of the solvent medium. Our results highlight the presence of a strong selectivity toward the monodentate binding mode on surface Ga atoms for both the carboxylic and phosphonic acid groups. Since the binding modes have a strong influence on the hole injection thermodynamics, these findings have direct implications for further development of delafossite based p-type DSSCs.
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http://dx.doi.org/10.3389/fchem.2019.00158DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6449920PMC
March 2019

Analysis of nascent silicon phase-change gratings induced by femtosecond laser irradiation in vacuum.

Sci Rep 2018 Aug 21;8(1):12498. Epub 2018 Aug 21.

Dipartimento di Fisica "Ettore Pancini", Università di Napoli Federico II, Complesso Universitario di Monte S. Angelo, Via Cintia, I-80126, Napoli, Italy.

The formation of periodic surface structures is a general effect of femtosecond laser irradiation of solid targets showing promising interest in material science and technology. However, the experiments are typically carried out in air, a condition in which the target surface becomes densely decorated with nanoparticles that can influence the formation of the surface structures in the early stage of the irradiation process. Here we report an investigation of structures generation on a silicon surface irradiated in vacuum (10 mbar) with a low number of laser pulses (N ≤ 10) that exploits several microscopy techniques (optical, atomic force, electron and Raman). Our analyses allow identifying the creation of silicon phase-change gratings consisting of alternating amorphous and crystalline periodic lines, with almost no material removal, located at the periphery of a shallow ablation crater. These gratings originate from two different kinds of defects: (i) the first is characterized by a peculiar lobed shape that is produced by the first few laser pulses; (ii) the second is provided by the one-dimensional, linear singularity defined by the ablation edge of the nascent crater. Both kind of defects lead to grating structures extending outwards the amorphous central area of the crater along the direction of the laser polarization. Comparative analysis with the surface formed in air, in the same experimental conditions, evidences the important role played by nanoparticles densely decorating the target in air and the striking variation occurring in vacuum.
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http://dx.doi.org/10.1038/s41598-018-30269-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6104067PMC
August 2018

Structural Basis of Polydopamine Film Formation: Probing 5,6-Dihydroxyindole-Based Eumelanin Type Units and the Porphyrin Issue.

ACS Appl Mater Interfaces 2018 Mar 29;10(9):7670-7680. Epub 2017 Sep 29.

Université de Strasbourg , Faculté de Chirurgie Dentaire , 8 rue Sainte Elisabeth , 67000 Strasbourg , France.

The role of 5,6-dihydroxyindole (DHI)-based oligomers, including porphyrin-like tetramers, in polydopamine (PDA) film formation was addressed by a comparative structural investigation against model polymers from DHI and its 2,7'-dimer. MALDI-MS data showed that (a) PDA is structurally different from DHI melanin and does not contain species compatible with DHI-based oligomers as primary building blocks; (b) PDA films and precipitate display a single main peak at m/ z 402 in common; (c) no species matching the range of m/ z values expected for cyclic porphyrin-type tetramers was detected in DHI melanin produced in the presence or in the absence of folic acid (FA) as templating agent, nor by oxidation of the 2,7'-dimer of DHI as putative precursor. N NMR resonances and Raman spectra predicted by extensive DFT calculations on porphyrin-type structures at various oxidation levels did not match spectral data for PDA or DHI melanin. Notably, unlike PDA, which gave structurally homogeneous films on quartz on atomic force microscopy (AFM) and micro-Raman spectroscopy, DHI melanin did not form any adhesive deposit after as long as 24 h. It is concluded that PDA film deposition involves structural components unrelated to DHI-based oligomers or porphyrin-type tetramers, which, on mechanism-based analysis, may arise by quinone-amine conjugation leading to polycyclic systems with extensive chain breakdown.
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http://dx.doi.org/10.1021/acsami.7b09662DOI Listing
March 2018

Light-Driven Wettability Tailoring of Azopolymer Surfaces with Reconfigured Three-Dimensional Posts.

ACS Appl Mater Interfaces 2017 Sep 23;9(35):30133-30142. Epub 2017 Aug 23.

Center for Nanoscale Systems, Harvard University , 9 Oxford Street, Cambridge, Massachusetts 02138, United States.

The directional light-induced mass migration phenomenon arising in the photoresponsive azobenzene-containing materials has become an increasingly used approach for the fabrication of controlled tridimensional superficial textures. In the present work we demonstrate the tailoring of the superficial wettability of an azopolymer by means of the light-driven reconfiguration of an array of imprinted micropillars. Few simple illumination parameters are controlled to induce nontrivial wetting effects. Wetting anisotropy with controlled directionality, unidirectional spreading, and even polarization-intensity driven two-dimensional paths for wetting anisotropy are obtained starting from a single pristine pillar geometry. The obtained results prove that the versatility of the light-reconfiguration process, together with the possibility of reversible reshaping at reduced costs, represents a valid approach for both applications and fundamental studies in the field of geometry-based wettability of solid surfaces.
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http://dx.doi.org/10.1021/acsami.7b08025DOI Listing
September 2017

New microscopy technique based on position localization of scattering particles.

Opt Express 2017 May;25(10):11530-11549

We introduce the Holographic - Single Scatterer Localization Microscopy in which we combine dynamical laser speckle illumination with centroid localization of backscattered light spots in order to localize isolated scattering particles. The reconstructed centroid images show very accurate particle localization, with precision much better than the width of diffraction-limited image of the particles recorded by the CCD. Furthermore, the method provides an improved resolution in distinguishing two very close scattering objects compared to the standard laser scanning techniques and can be assimilated to a confocal technique in the ability of light background rejection in three-dimensional disposition of scattering objects. The illumination is controlled via a digital holography setup based on the use of a spatial light modulator. This allows not only a high level of versatility in the illumination patterns, but also the remarkable characteristics of absence of moving mechanical parts, typical of the laser scanning techniques, and the possibility of strongly miniaturizing the setup.
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http://dx.doi.org/10.1364/OE.25.011530DOI Listing
May 2017

Giant O-Induced Photoluminescence Modulation in Hierarchical Titanium Dioxide Nanostructures.

ACS Sens 2017 Jan 4;2(1):61-68. Epub 2017 Jan 4.

Institute of Applied Sciences and Intelligent Systems (CNR-ISASI) , Via Campi Flegrei 34, I-80078 Pozzuoli, Italy.

We demonstrate exceptionally large modulation of PL intensity in hierarchical titanium dioxide (TiO) nanostructures exposed to molecular oxygen (O). Optical responsivities up to about 1100% at 20% O concentrations are observed in hyperbranched anatase-phase hierarchical structures, outperforming those obtainable by commercial TiO 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 TiO or other MOXs and open interesting scenarios toward the development of all-optical and/or hybrid (opto/electrical) chemical sensors with improved sensitivity.
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http://dx.doi.org/10.1021/acssensors.6b00432DOI Listing
January 2017

Anomalous evolution of broadband optical absorption reveals dynamic solid state reorganization during eumelanin build-up in thin films.

Sci Rep 2017 03 31;7(1):522. Epub 2017 Mar 31.

INFN, Sezione di Napoli, Via Cintia, 80126, Napoli, Italy.

The origin of eumelanin optical properties remains a formidable conundrum preventing a detailed understanding of the complex photo-protective role of these widespread natural pigments and the rational design of innovative bioinspired materials for optoelectronic applications. Here we report the unusual kinetic and thickness-dependent evolution of the optical properties of black eumelanin polymers generated by spontaneous aerial polymerization of 5,6-dihydroxyindole (DHI) thin films (0.1-1 μm), consistent with peculiar solid state reorganization mechanisms governing broadband absorption. The complete reversal of eumelanin UV-visible transmittance spectrum curvature on passing from 0.2 to 0.5 μm thick films, the marked increase in visible extinction coefficients with increasing film thickness and the higher UV extinction coefficients in slowly vs. rapidly generated polymers concur to support distinct dynamic regimes of solid-state molecular reorganization at the nanoscale level and to do affect the development of broadband visible absorption. Solid state control of molecular reorganization disclosed herein may delineate new rational strategies for tuning optical properties in eumelanin thin films for optoelectronic applications.
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http://dx.doi.org/10.1038/s41598-017-00597-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5428701PMC
March 2017

Spin-to-orbital angular momentum conversion in dielectric metasurfaces: erratum.

Opt Express 2017 02;25(4):4239

We would like to clarify our paper [Opt. Express25, 377 (2017)] abstract sentence "These beams carry orbital angular momentum proportional to the number of intertwined helices constituting the wavefront."
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http://dx.doi.org/10.1364/OE.25.004239DOI Listing
February 2017

Spin-to-orbital angular momentum conversion in dielectric metasurfaces.

Opt Express 2017 Jan;25(1):377-393

Vortex beams are characterized by a helical wavefront and a phase singularity point on the propagation axis that results in a doughnut-like intensity profile. These beams carry orbital angular momentum proportional to the number of intertwined helices constituting the wavefront. Vortex beams have many applications in optics, such as optical trapping, quantum optics and microscopy. Although beams with such characteristics can be generated holographically, spin-to-orbital angular momentum conversion has attracted considerable interest as a tool to create vortex beams. In this process, the geometrical phase is exploited to create helical beams whose handedness is determined by the circular polarization (left/right) of the incident light, that is by its spin. Here we demonstrate high-efficiency Spin-to-Orbital angular momentum-Converters (SOCs) at visible wavelengths based on dielectric metasurfaces. With these SOCs we generate vortex beams with high and fractional topological charge and show for the first time the simultaneous generation of collinear helical beams with different and arbitrary orbital angular momentum. This versatile method of creating vortex beams, which circumvents the limitations of liquid crystal SOCs and adds new functionalities, should significantly expand the applications of these beams.
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http://dx.doi.org/10.1364/OE.25.000377DOI Listing
January 2017

Observation of a photoinduced, resonant tunneling effect in a carbon nanotube-silicon heterojunction.

Beilstein J Nanotechnol 2015 10;6:704-10. Epub 2015 Mar 10.

INFN, Sezione di Bari and Dipartimento di Fisica, Università degli Studi di Bari, Via Amendola 173, 70126 Bari, Italy.

A significant resonant tunneling effect has been observed under the 2.4 V junction threshold in a large area, carbon nanotube-silicon (CNT-Si) heterojunction obtained by growing a continuous layer of multiwall carbon nanotubes on an n-doped silicon substrate. The multiwall carbon nanostructures were grown by a chemical vapor deposition (CVD) technique on a 60 nm thick, silicon nitride layer, deposited on an n-type Si substrate. The heterojunction characteristics were intensively studied on different substrates, resulting in high photoresponsivity with a large reverse photocurrent plateau. In this paper, we report on the photoresponsivity characteristics of the device, the heterojunction threshold and the tunnel-like effect observed as a function of applied voltage and excitation wavelength. The experiments are performed in the near-ultraviolet to near-infrared wavelength range. The high conversion efficiency of light radiation into photoelectrons observed with the presented layout allows the device to be used as a large area photodetector with very low, intrinsic dark current and noise.
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http://dx.doi.org/10.3762/bjnano.6.71DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4362292PMC
March 2015

Instability-induced pattern formation of photoactivated functional polymers.

Proc Natl Acad Sci U S A 2014 Dec 17;111(48):17017-22. Epub 2014 Nov 17.

School of Engineering and Applied Science, Harvard University, Cambridge, MA 02138;

Since the pioneering work of Turing on the formation principles of animal coat patterns [Turing AM (1952) Phil Trans R Soc Lond B 237(641):37-72], such as the stripes of a tiger, great effort has been made to understand and explain various phenomena of self-assembly and pattern formation. Prominent examples are the spontaneous demixing in emulsions, such as mixtures of water and oil [Herzig EM, et al. (2007) Nat Mater 6:966-971]; the distribution of matter in the universe [Kibble TWB (1976) J Phys A: Math Gen 9(8):1387]; surface reconstruction in ionic crystals [Clark KW, et al. (2012) Nanotechnol 23(18):185306]; and the pattern formation caused by phase transitions in metal alloys, polymer mixtures and binary Bose-Einstein condensates [Sabbatini J, et al. (2011) Phys Rev Lett 107:230402]. Photoactivated pattern formation in functional polymers has attracted major interest due to its potential applications in molecular electronics and photoresponsive systems. Here we demonstrate that photoactivated pattern formation on azobenzene-containing polymer films can be entirely explained by the physical concept of phase separation. Using experiments and simulations, we show that phase separation is caused by an instability created by the photoactivated transitions between two immiscible states of the polymer. In addition, we have shown in accordance with theory, that polarized light has a striking effect on pattern formation indicated by enhanced phase separation.
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http://dx.doi.org/10.1073/pnas.1409718111DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4260583PMC
December 2014

Molecular model for light-driven spiral mass transport in azopolymer films.

Phys Rev Lett 2013 Apr 5;110(14):146102. Epub 2013 Apr 5.

CNR-SPIN, Complesso Universitario di Monte Sant'Angelo, 80126 Napoli, Italy and Dipartimento di Fisica, Università di Napoli "Federico II," Complesso Universitario di Monte Sant'Angelo, 80126 Napoli, Italy.

Azopolymer films exposed to nonuniform illumination exhibit a phenomenon of light-induced mass transport, leading to the formation of permanent relief patterns on the film surface. Its underlying microscopic mechanism remains unclear, despite many years of research effort. Here we introduce a model of the mass migration process based on anisotropic light-driven molecular diffusion. A key ingredient of our model is an enhanced molecular diffusion in proximity of the free polymer surface, which is essential for explaining, in particular, the recently observed spiral-shaped reliefs resulting from vortex-beam illumination.
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http://dx.doi.org/10.1103/PhysRevLett.110.146102DOI Listing
April 2013

Light-induced spiral mass transport in azo-polymer films under vortex-beam illumination.

Nat Commun 2012 ;3:989

CNR-SPIN and Dipartimento di Scienze Fisiche, Università degli Studi di Napoli Federico II, Via Cintia, Napoli 80126, Italy.

When an azobenzene-containing polymer film is exposed to non-uniform illumination, a light-induced mass migration process may be induced, leading to the formation of relief patterns on the polymer-free surface. Despite many years of research effort, several aspects of this phenomenon remain poorly understood. Here we report the appearance of spiral-shaped relief patterns on the polymer film under the illumination of focused Laguerre-Gauss beams with helical wavefronts and an optical vortex at their axis. The induced spiral reliefs are sensitive to the vortex topological charge and to the wavefront handedness. These findings are unexpected because the doughnut-shaped intensity profile of Laguerre-Gauss beams contains no information about the wavefront handedness. We propose a model that explains the main features of this phenomenon through the surface-mediated interference of the longitudinal and transverse components of the optical field. These results may find applications in optical nanolithography and optical-field nanoimaging.
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http://dx.doi.org/10.1038/ncomms1996DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3432464PMC
November 2012

Silicon infrared diffuser for wireless communication.

Appl Opt 2006 Sep;45(26):6746-9

Research Center Ente Nazionale Energia e Ambiente, Portici (Napoli), Italy.

We show what we believe to be a novel way to use silicon in infrared radio communication as a suitable material for the realization of optical diffusers in the range of 850-1600 nm. A crystalline silicon wafer is made porous by means of electrochemical etching. The porous silicon produced is optically characterized, and measurements report a high reflectance in the band of interest. We also study the angular distribution of diffused radiation by the porous silicon surface at different angles of incident radiation. Measurements show that radiation diffuses in a quasi-Lambertian manner, confirming the good performance of this material as an incident radiation diffuser.
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http://dx.doi.org/10.1364/ao.45.006746DOI Listing
September 2006
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