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The possibility to increase the efficiency of photovoltaic (PV) cells based on hybrid carbon nanotube (CNT)–Si heterojunctions is related to the ability to control the chemical properties of the CNT–Si interface and of the CNT bundle layer. In spite of the encouraging performances of PV cells based on multiwall (MW) CNT, so far few efforts have been made in the study of this device compared to single wall (SW) CNT–Si interfaces. Here, surface and interface effects on the current–voltage characteristic curves of MW CNT–Si hybrid junctions are investigated through exposure to HF vapors and to 10 ppm-NO2 and compared to the effects detected in SW CNT–Si junctions. Quite similar results in terms of open circuit voltage, short circuit current density, and efficiency are found for both cells, suggesting that exposure to HF vapors mostly affects the interface chemical properties, i.e., the silicon oxidation state, that in both junctions reach an optimal state about 50 h after etching. In turn, NO2 exposure has larger effects on the SW-based cell, consistently with the larger surface-to-volume ratio of SW with respect to MW. In both cases, the efficiency value reaches a maximum after 28 min, before dropping when the NO2 molecules desorb from the surface. A combined analysis of current–voltage curves and photoemission data collected along the different phases of gas exposures allowed us to relate changes in the electrical properties to the chemistry of Si at the interface.

Sound is usually generated in a medium by an electromechanical vibrating structure. The geometrical size and inertia of the structure set the frequency cutoff in the sound-transduction mechanism and, often, different vibrating structures are necessary to cover the whole range from infrasound to ultrasound. An alternative mechanism without any physical movement of the emitter is the thermoacoustic effect, where sound is produced by Joule heating in a conductive material. Here we show that a single thermoacoustic transducer based on a graphene aerogel can emit ultrabroadband sound from infrasound (1 Hz) to ultrasound (20 MHz), with no harmonic distortion. Since conventional acoustic transducers are frequency band limited due to their transduction mechanism, ultrabroadband graphene aerogels may offer a valid alternative to conventional hi-fi loudspeakers, and infrasound and ultrasound transducers., 9 pages, 7 figures. arXiv admin note: text overlap with arXiv:1909.08858

Graphene hydrophobic coatings paved the way towards a new generation of optoelectronic and fluidic devices. Nevertheless, such hydrophobic thin films rely only on graphene non-polar surface, rather than taking advantage of its surface roughness. Furthermore, graphene is typically not self-standing. Differently, carbon aerogels have high porosity, large effective surface area due to their surface roughness, and very low mass density, which make them a promising candidate as a super-hydrophobic material for novel technological applications. However, despite a few works reporting the general super-hydrophobic and lipophilic behavior of the carbon aerogels, a detailed characterization of their wetting properties is still missing, to date. Here, the wetting properties of graphene aerogels are demonstrated in detail. Without any chemical functionalization or patterning of their surface, the samples exhibit a super-lipophilic state and a stationary super-hydrophobic state with a contact angle up to $150\pm15^{\deg}$ and low contact angle hysteresis $\approx15^{\deg}$, owing to the fakir effect. In addition, the adhesion force of the graphene aerogels in contact with the water droplets and their surface tension are evaluated. For instance, the unique wettability and enhanced liquid absorption of the graphene aerogels can be exploited for reducing contamination from oil spills and chemical leakage accidents., Comment: 7 pages, 4 figures

Metamaterials have recently established a new paradigm for enhanced light absorption in state-of-the-art photodetectors. Here, we demonstrate broadband, highly efficient, polarization-insensitive, and gate-tunable photodetection at room temperature in a novel metadevice based on gold/graphene Sierpinski carpet plasmonic fractals. We observed an unprecedented internal quantum efficiency up to 100% from the near-infrared to the visible range with an upper bound of optical detectivity of $10^{11}$ Jones and a gain up to $10^{6}$, which is a fingerprint of multiple hot carriers photogenerated in graphene. Also, we show a 100-fold enhanced photodetection due to highly focused (up to a record factor of $|E/E_{0}|\approx20$ for graphene) electromagnetic fields induced by electrically tunable multimodal plasmons, spatially localized in self-similar fashion on the metasurface. Our findings give direct insight into the physical processes governing graphene plasmonic fractal metamaterials. The proposed structure represents a promising route for the realization of a broadband, compact, and active platform for future optoelectronic devices including multiband bio/chemical and light sensors., Comment: 10 pages, 6 figures

We observed a 73% enhancement of the power conversion efficiency (PCE) of a photovoltaic cell based on a single wall carbon nanotube/Si hybrid junction after exposing the device to a limited amount (10 ppm) of NO2 diluted in dry air. On the basis of a computational modeling of the junction, this enhancement is discussed in terms of both carbon nanotube (CNT) p-doping, induced by the interaction with the oxidizing molecules, and work function changes across the junction. Unlike studies so far reported, where the PCE enhancement was correlated only qualitatively to CNT doping, our study (i) provides a novel and reversible path to tune and considerably enhance the cell efficiency by a few ppm gas exposure, and (ii) shows computational results that quantitatively relate the observed effects to the electrostatics of the cell through a systematic calculation of the work function. These effects have been cross-checked by exposing the cell to reducing molecules (i.e·NH3) that resulted to be detrimental to the cell efficiency, consistently with the theoretical ab-initio calculations.

Optical anti-reflection is achieved in natural surfaces by exploiting hierarchical surface morphology. Here, we show that single-walled carbon nanotube (SWCNT) coatings deposited on silicon (Si) realize a broad-band, omnidirectional, and nearly polarization-independent suppression of Si optical reflection, with an increase of film absorption. This is attributed to a biomimetic, hierarchical surface morphology, which introduces a graded refractive index–the so-called moth-eye effect. Moreover, the anti-reflective behavior can be tuned by varying the SWCNT film thickness. The SWCNT random networks are realized by a simple, rapid, reproducible, and inexpensive solution-processing technique and deposited on Si by a dry-transfer printing method, at room temperature. The technology may be used to coat arbitrary substrates such as optical instruments, radiometric applications, light and thermal sensors, solar cells, and light emitting diodes; thus improving the device absorption or emission of light, due to the film optical properties.

Metamaterials have recently established a new paradigm for enhanced light absorption in state-of-the-art photodetectors. Here, we demonstrate broadband, highly efficient, polarization-insensitive, and gate-tunable photodetection at room temperature in a novel metadevice based on gold/graphene Sierpinski carpet plasmonic fractals. We observed an unprecedented internal quantum efficiency up to 100% from the near-infrared to the visible range with an upper bound of optical detectivity of 10

In this paper, we show that it is possible to synthesize carbon-based three-dimensional networks by adding sulfur, as growth enhancer, during the synthesis process. The obtained material is self-supporting and consists of curved and interconnected carbon nanotubes and to lesser extent of carbon fibers. Studies on the microstructure indicate that the assembly presents a marked variability in the tube external diameter and in the inner structure. We study the relationship between the observed microscopic properties and some potential applications. In particular, we show that the porous nature of the network is directly responsible for the hydrophobic and the lipophilic behavior. Moreover, we used a cut piece of the produced carbon material as working electrode in a standard electrochemical cell and, thus, demonstrating the capability of the system to respond to incident light in the visible and near-ultraviolet region and to generate a photocurrent.

Self-assembled hierarchical solid surfaces are very interesting for wetting phenomena, as observed in a variety of natural and artificial surfaces. Here, we report single-walled (SWCNT) and multi-walled carbon nanotube (MWCNT) thin films realized by a simple, rapid, reproducible, and inexpensive filtration process from an aqueous dispersion, that was deposited at room temperature by a dry-transfer printing method on glass. Furthermore, the investigation of carbon nanotube films through scanning electron microscopy (SEM) reveals the multi-scale hierarchical morphology of the self-assembled carbon nanotube random networks. Moreover, contact angle measurements show that hierarchical SWCNT/MWCNT composite surfaces exhibit a higher hydrophobicity (contact angles of up to 137{\deg}) than bare SWCNT (110{\deg}) and MWCNT (97{\deg}) coatings, thereby confirming the enhancement produced by the surface hierarchical morphology., Comment: 7 pages, 5 figures, This article is part of the Thematic Series "Self-assembly of nanostructures and nanomaterials"

Ultrablack metamaterials are artificial materials able to harvest all the incident light regardless of wavelength, angle, or polarization. Here, we show the ultrablack properties of randomly aligned hierarchical carbon nanotube arrays with thicknesses below 200 nm. The thin coatings are realized by solution processing and dry-transfer deposition on different substrates. The hierarchical surface morphology of the coatings is biomimetic and provides a large effective area that improves the film optical absorption. Also, such a morphology is responsible for the moth-eye effect, which leads to the omnidirectional and polarization-independent suppression of optical reflection. The films exhibit an emissivity up to 99.36% typical of an ideal black body, resulting in the thinnest ultrablack metamaterial ever reported. Such a material may be exploited for thermal, optical, and optoelectronic devices such as heat sinks, optical shields, solar cells, light and thermal sensors, and light-emitting diodes.

Bulk heterojunction films made of polychiral single-walled carbon nanotubes (SWCNTs) form efficient heterojunction solar cells with n-type crystalline silicon (n-Si), due to their superior electronic, optical, and electrical properties. The films are multi-functional, since their hierarchical surface morphology provides a biomimetical anti-reflective, air-stable, and hydrophobic encapsulation for Si. Also, the films have a large effective area conferring them high optical absorption, which actively contribute to the solar energy harvesting together with Si. Here, we report photovoltaic devices with photoconversion efficiency up to 12% and a record 100% internal quantum efficiency (IQE). Such unprecedented IQE value is truly remarkable and indicates that every absorbed photon from the device, at some wavelengths, generates a pair of separated charge carriers, which are collected at the electrodes. The SWCNT/Si devices favor high and broadband carrier photogeneration; charge dissociation of ultra-fast hot excitons; transport of electrons through n-Si and high-mobility holes through the SWCNT percolative network. Moreover, by varying the film thickness, it is possible tailoring the physical properties of such a two-dimensional interacting system, therefore the overall device features. These results not only pave the way for low-cost, high-efficient, and broadband photovoltaics, but also are promising for the development of generic SWCNT-based optoelectronic applications.

Multi-walled carbon nanotube (MWCNT) films form efficient heterojunction solar cells with n-type crystalline silicon (n-Si), due to their superior optical and electrical properties. Here, we report air-stable photovoltaic devices with record photoconversion efficiency of 10%. We realized thin films consisting of MWCNTs arranged in semitransparent random networks deposited on n-Si substrates by a simple, rapid, reproducible, and inexpensive vacuum filtration process at room temperature. Such heterojunctions favor high and broadband carrier photogeneration, extending the Si spectral response from near infrared to near ultraviolet range; charge dissociation of ultrafast hot carriers [1]; transport of electrons through n-Si and high-mobility [2] holes through the MWCNT percolative network. Furthermore, by varying the MWCNT film thickness, it is possible tailoring its optical and electrical properties, therefore the overall device optoelectronic features. These results not only pave the way for low-cost, efficient, and broadband photovoltaics, but also are promising for the development of MWCNT-based optoelectronic applications.

Here, we investigate the thickness of single-walled (SWCNT) and multi-walled carbon nanotube (MWCNT) random network films by angle-resolved X-ray photoemission spectroscopy. Furthermore, we estimate the absorption coefficient of carbon nanotube (CNT) films through the Lambert–Beer law, by measuring film optical spectra. Moreover, the knowledge of the absorption coefficient provides an easier, reliable, and faster method of investigation for generic CNT film thickness. In addition, the absorption coefficient leads to the information of the absorption length for SWCNT and MWCNT films, which is a physical quantity of fundamental interest for optoelectronic applications, such as light emitting diodes, photovoltaics, and in general light absorbers.

A hierarchical structure is an assembly with a multi-scale morphology and with a large and accessible surface area. Recent advances in nanomaterial science have made increasingly possible the design of hierarchical surfaces with specific and tunable properties. Here, we report the fractal analysis of hierarchical single-walled carbon nanotube (SWCNT) films realized by a simple, rapid, reproducible and inexpensive filtration process from an aqueous dispersion, then deposited by drytransfer printing method on several substrates, at room temperature. Furthermore, by varying the thickness of carbon nanotube random networks, it is possible tailoring their wettability due to capillary phenomena in the porous films. Moreover, in order to describe the wetting properties of such surfaces, we introduce a two-dimensional extension of the Wenzel-Cassie-Baxter theory. The hierarchical surface roughness of SWCNT coatings coupled with their exceptional and tunable optical and electrical properties provide an ideal hydrophobic composite surface for a new class of optoelectronic and nanofluidic devices.

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.

We report on the theoretical analysis of bosonic and fermionic non-interacting systems in a discrete two-particle quantum walk affected by different kinds of disorder. We considered up to 100-step QWs with a spatial, temporal and space-temporal disorder observing how the randomness and the wavefunction symmetry non-trivially affect the final spatial probability distribution, the transport properties and the Shannon entropy of the walkers., Comment: 13 pages, 10 figures. arXiv admin note: text overlap with arXiv:1101.2638 by other authors

Waves fail to propagate in random media. First predicted for quantum particles in the presence of a disordered potential, Anderson localization has been observed also in classical acoustics, electromagnetism and optics. Here, for the first time, we report the observation of Anderson localization of pairs of entangled photons in a two-particle discrete quantum walk affected by position dependent disorder. A quantum walk on a disordered lattice is realized by an integrated array of interferometers fabricated in glass by femtosecond laser writing. A novel technique is used to introduce a controlled phase shift into each unit mesh of the network. Polarization entanglement is exploited to simulate the different symmetries of the two-walker system. We are thus able to experimentally investigate the genuine effect of (bosonic and fermionic) statistics in the absence of interaction between the particles. We will show how different types of randomness and the symmetry of the wave-function affect the localization of the entangled walkers., Comment: 7 pages, 5 figures, revised version published on Nature Photonics 7, 322-328 (2013)

We have taken advantage of the native surface roughness and the iron content of AISI 316 stainless steel to direct grow multi-walled carbon nanotube (MWCNT) random networks by chemical vapor deposition (CVD) at low-temperature ($< 1000^{\circ}$C), without the addition of any external catalysts or time-consuming pre-treatments. In this way, super-hydrophobic MWCNT films on stainless steel sheets were obtained, exhibiting high contact angle values ($154^{\circ}$) and high adhesion force (high contact angle hysteresis). Furthermore, the investigation of MWCNT films at scanning electron microscopy (SEM) reveals a two-fold hierarchical morphology of the MWCNT random networks made of hydrophilic carbonaceous nanostructures on the tip of hydrophobic MWCNTs. Owing to the Salvinia effect, the hydrophobic and hydrophilic composite surface of the MWCNT films supplies a stationary super-hydrophobic coating for conductive stainless steel. This biomimetical inspired surface not only may prevent corrosion and fouling but also could provide low-friction and drag-reduction., 6 pages, 3 figures

Multi-walled carbon nanotube (MWCNT) films form efficient heterojunction solar cells with n-type crystalline silicon (n-Si), due to their superior optical and electrical properties. Here, we report air-stable photovoltaic devices with record photoconversion efficiency of 10%. We realized thin films consisting of MWCNTs arranged in semitransparent random networks deposited on n-Si substrates by a simple, rapid, reproducible, and inexpensive vacuum filtration process at room temperature. Such heterojunctions favor high and broadband carrier photogeneration, extending the Si spectral response from near infrared to near ultraviolet range; charge dissociation of ultrafast hot carriers [1]; transport of electrons through n-Si and high-mobility [2] holes through the MWCNT percolative network. Furthermore, by varying the MWCNT film thickness, it is possible tailoring its optical and electrical properties, therefore the overall device optoelectronic features. These results not only pave the way for low-cost, efficient, and broadband photovoltaics, but also are promising for the development of MWCNT-based optoelectronic applications.

The ability to generate, amplify, mix, and modulate sound with no harmonic distortion in a passive opto-acoustic device would revolutionize the field of acoustics. The photo-thermo-acoustic (PTA) effect allows to transduce light into sound without any bulk electro-mechanically moving parts and electrical connections, as for conventional loudspeakers. Also, PTA devices can be integrated with standard silicon complementary metal-oxide semiconductor (CMOS) fabrication techniques. Here, we demonstrate that the ultimate PTA efficiency of graphene aerogels, depending on their particular thermal and optical properties, can be experimentally achieved by reducing their mass density. Furthermore, we illustrate that the aerogels behave as an omnidirectional point-source throughout the audible range with no harmonic distortion. This research represents a breakthrough for audio-visual consumer technologies and it could pave the way to novel opto-acoustic sensing devices., 6 pages, 4 figures

Il contributo indaga l'attuale stato dell'arte della dantologia slovena nonché la ricezione traduttiva, editoriale, storicoletteraria, critica e culturale di Dante Alighieri tra gli sloveni, con stringati accenni alle sue più recenti fortune letterarie anche tra i restanti slavi meridionali.