Nanometric Moiré Stripes on the Surface of BiSe Topological Insulator.

Mismatch between adjacent atomic layers in low-dimensional materials, generating moiré patterns, has recently emerged as a suitable method to tune electronic properties by inducing strong electron correlations and generating novel phenomena. Beyond graphene, van der Waals structures such as three-dimensional (3D) topological insulators (TIs) appear as ideal candidates for the study of these phenomena due to the weak coupling between layers. Here we discover and investigate the origin of 1D moiré stripes on the surface of BiSe TI thin films and nanobelts.

Van der Waals Heteroepitaxy of Air-Stable Quasi-Free-Standing Silicene Layers on CVD Epitaxial Graphene/6H-SiC.

Graphene, consisting of an inert, thermally stable material with an atomically flat, dangling-bond-free surface, is by essence an ideal template layer for van der Waals heteroepitaxy of two-dimensional materials such as silicene. However, depending on the synthesis method and growth parameters, graphene (Gr) substrates could exhibit, on a single sample, various surface structures, thicknesses, defects, and step heights. These structures noticeably affect the growth mode of epitaxial layers, .

Large-area, high-responsivity, fast and broadband graphene/n-Si photodetector.

A graphene/Si heterojunction device has been realized to overcome many different requests necessary to make it a versatile, widely used and competitive detector. The obtained photodetectors, which operate at room temperature, are sensitive in the spectral region from ultraviolet (240 nm) to infrared (2000 nm) and they can be used in different configurations that allow a high responsivity up to 10 A W, a rise time of a few nanoseconds, an external quantum efficiency greater than 300%, and a linear response for different light sources. This is allowed by the high quality of the graphene deposited on a large area of 8 mm, and by the interdigitated design of the contacts, both preserving the excellent properties of graphene when switching from nanoscale to macroscopic dimensions of commonly used devices.

Gas Sensing with Solar Cells: The Case of NH Detection through Nanocarbon/Silicon Hybrid Heterojunctions.

Photovoltaic (PV) cells based on single-walled carbon nanotube (SWCNT)/silicon (Si) and multiwalled carbon nanotube (MWCNT)/Si junctions were tested under exposure to NH in the 0-21 ppm concentration range. The PV cell parameters remarkably changed upon NH exposure, suggesting that these junctions, while being operated as PV cells, can react to changes in the environment, thereby acting as NH gas sensors. Indeed, by choosing the open-circuit voltage, V, parameter as read-out, it was found that these cells behaved as gas sensors, operating at room temperature with a response higher than chemiresistors developed on the same layers.

Functional rewiring across spinal injuries via biomimetic nanofiber scaffolds.

The regrowth of severed axons is fundamental to reestablish motor control after spinal-cord injury (SCI). Ongoing efforts to promote axonal regeneration after SCI have involved multiple strategies that have been only partially successful. Our study introduces an artificial carbon-nanotube based scaffold that, once implanted in SCI rats, improves motor function recovery.

Stoichiometric BiSe topological insulator ultra-thin films obtained through a new fabrication process for optoelectronic applications.

A new fabrication process is developed for growing BiSe topological insulators in the form of nanowires/nanobelts and ultra-thin films. It consists of two consecutive procedures: first BiSe nanowires/nanobelts are deposited by standard catalyst free vapour-solid deposition on different substrates positioned inside a quartz tube. Then, the BiSe, stuck on the inner surface of the quartz tube, is re-evaporated and deposited in the form of ultra-thin films on new substrates at a temperature below 100 °C, which is of relevance for flexible electronic applications.

Dramatic efficiency boost of single-walled carbon nanotube-silicon hybrid solar cells through exposure to ppm nitrogen dioxide in air: An ab-initio assessment of the measured device performances.

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 NO 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.

Coating ZnO nanoparticle films with DNA nanolayers for enhancing the electron extracting properties and performance of polymer solar cells.

Here we present for the first time polymer solar cells that incorporate biological material that show state of the art efficiencies in excess of 8%. The performance of inverted polymer solar cells was improved significantly after deposition of ZnO nanoparticles (ZnO-NPs) together with a thin deoxyribonucleic acid nanolayer and used as an electron extraction layer (EEL). The ZnO-NPs/DNA double layer improved the rectifying ratio, shunt resistance of the cells as well as lowering the work function of the electron-collecting contact.

Structural and photoluminescence properties of silicon nanowires extracted by means of a centrifugation process from plasma torch synthesized silicon nanopowder.

We report on a method for the extraction of silicon nanowires (SiNWs) from the by-product of a plasma torch based spheroidization process of silicon. This by-product is a nanopowder which consists of a mixture of SiNWs and silicon particles. By optimizing a centrifugation based process, we were able to extract substantial amounts of highly pure Si nanomaterials (mainly SiNWs and Si nanospheres (SiNSs)).

Hybridized C-O-Si Interface States at the Origin of Efficiency Improvement in CNT/Si Solar Cells.

Despite the astonishing values of the power conversion efficiency reached, in just less than a decade, by the carbon nanotube/silicon (CNT/Si) solar cells, many doubts remain on the underlying transport mechanisms across the CNT/Si heterojunction. Here, by combining transient optical spectroscopy in the femtosecond timescale, X-ray photoemission, and a systematic tracking of I-V curves across all phases of the interlayer SiO growth at the interface, we grasp the mechanism that adequately preserves charge separation at the junction, hindering the photoexcited carrier recombination. Moreover, supported by ab initio calculations aimed to model the complex CNT-Si heterointerface, we show that oxygen-related states at the interface act as entrapping centers for the photoexcited electrons, thus preventing recombination with holes that can flow from Si to CNT across the SiO layer.

Self-assembly of silicon nanowires studied by advanced transmission electron microscopy.

Scanning transmission electron microscopy (STEM) was successfully applied to the analysis of silicon nanowires (SiNWs) that were self-assembled during an inductively coupled plasma (ICP) process. The ICP-synthesized SiNWs were found to present a Si-SiO core-shell structure and length varying from ≈100 nm to 2-3 μm. The shorter SiNWs (maximum length ≈300 nm) were generally found to possess a nanoparticle at their tip.

Formation of Silicene Nanosheets on Graphite.

The extraordinary properties of graphene have spurred huge interest in the experimental realization of a two-dimensional honeycomb lattice of silicon, namely, silicene. However, its synthesis on supporting substrates remains a challenging issue. Recently, strong doubts against the possibility of synthesizing silicene on metallic substrates have been brought forward because of the non-negligible interaction between silicon and metal atoms.

van der Waals Heteroepitaxy of Germanene Islands on Graphite.

We fabricated flat, two-dimensional germanium sheets showing a honeycomb lattice that matches that of germanene by depositing submonolayers of Ge on graphite at room temperature and subsequent annealing to 350 °C. Scanning tunneling microscopy shows that the germanene islands have a small buckling with no atomic reconstruction and does not give any hints for alloy formation and hybridization with the substrate. Our density functional theory calculations of the structural properties agree well with our experimental findings and indicate that the germanene sheet interacts only weakly with the substrate underneath.

3D meshes of carbon nanotubes guide functional reconnection of segregated spinal explants.

In modern neuroscience, significant progress in developing structural scaffolds integrated with the brain is provided by the increasing use of nanomaterials. We show that a multiwalled carbon nanotube self-standing framework, consisting of a three-dimensional (3D) mesh of interconnected, conductive, pure carbon nanotubes, can guide the formation of neural webs in vitro where the spontaneous regrowth of neurite bundles is molded into a dense random net. This morphology of the fiber regrowth shaped by the 3D structure supports the successful reconnection of segregated spinal cord segments.

Peptide flatlandia: a new-concept peptide for positioning of electroactive probes in proximity to a metal surface.

A helical hexapeptide was designed to link in a rigid parallel orientation to a gold surface. The peptide sequence of the newly synthesized compound is characterized by the presence of two 4-amino-1,2-dithiolane-4-carboxylic acid (Adt) residues (positions 1 and 4) to promote a bidentate interaction with the gold surface, two L-Ala residues (positions 2 and 5) and two-aminoisobutyric acid (Aib) residues (positions 3 and 6) to favor a high population of the 310-helix conformation. Furthermore, a ferrocenoyl (Fc) probe was inserted at the N-terminus to investigate the electronic conduction properties of the peptide.

Applications of three-dimensional carbon nanotube networks.

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.

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

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.

Exploiting the hierarchical morphology of single-walled and multi-walled carbon nanotube films for highly hydrophobic coatings.

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.

Super-hydrophobic multi-walled carbon nanotube coatings for stainless steel.

We have taken advantage of the native surface roughness and the iron content of AISI 316 stainless steel to directly grow multi-walled carbon nanotube (MWCNT) random networks by chemical vapor deposition (CVD) at low-temperature (1000°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°C) and high adhesion force (high contact angle hysteresis). Furthermore, the investigation of MWCNT films with 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.

Multi-fractal hierarchy of single-walled carbon nanotube hydrophobic coatings.

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.

Structural, electronic and photovoltaic characterization of multiwalled carbon nanotubes grown directly on stainless steel.

We have taken advantage of the native surface roughness and the iron content of AISI-316 stainless steel to grow multiwalled carbon nanotubes (MWCNTs) by chemical vapour deposition without the addition of an external catalyst. The structural and electronic properties of the synthesized carbon nanostructures have been investigated by a range of electron microscopy and spectroscopy techniques. The results show the good quality and the high graphitization degree of the synthesized MWCNTs.

Si nanotubes and nanospheres with two-dimensional polycrystalline walls.

We report on the characteristics of a new class of Si-based nanotubes and spherical nanoparticles synthesized by the dc-arc plasma method in a mixture of argon and hydrogen. These two nanostructures share common properties: they are hollow and possess very thin, highly polycrystalline and mainly oxidized walls. In particular, we get several hints indicating that their walls could constitute only one single Si oxidized layer.

Photocurrent generation in Ge nanocrystal/Si systems.

We report on the generation of photocurrent in the visible and ultraviolet range from planar devices built from the Ge nanocrystals grown on a heavy n-doped Si(001) substrate covered with 5 nm thick thermally grown SiO2. These Ge nanostructures/SiO2/n(+)-Si devices are shown to generate photocurrent with an Incident-Photon-to-electron Conversion Efficiency (IPCE) spectral range depending on the Ge nanocrystals size. The increase of the IPCE value of our devices in the 350-600 nm range correlates well with the absorbance of Ge.

Light harvesting with multiwall carbon nanotube/silicon heterojunctions.

We report on a significant photocurrent generation from a planar device obtained by coating a bare n doped silicon substrate with a random network of multiwall carbon nanotubes (MWCNTs). This MWCNT/n-Si hybrid device exhibits an incident photon to current efficiency reaching up to 34% at 670 nm. We also show that MWCNTs covering a quartz substrate still exhibit photocurrent, though well below than that of the MWCNTs coating the silicon substrate.