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 107 A W-1, 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 mm2, 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.

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)). While the purified SiNWs were found to have typical outer diameters in the 10-15 nm range and lengths of up to several µm, the SiNSs have external diameters in the 10-100 nm range. Interestingly, the SiNWs are found to have a thinner Si core (2-5 nm diam.) and an outer silicon oxide shell (with a typical thickness of ∼5-10 nm). High resolution transmission electron microscopy (HRTEM) observations revealed that many SiNWs have a continuous cylindrical core, whereas others feature a discontinuous core consisting of a chain of Si nanocrystals forming a sort of 'chaplet-like' structures. These plasma-torch-produced SiNWs are highly pure with no trace of any metal catalyst, suggesting that they mostly form through SiO-catalyzed growth scheme rather than from metal-catalyzed path. The extracted Si nanostructures are shown to exhibit a strong photoluminescence (PL) which is found to blue-shift from 950 to 680 nm as the core size of the Si nanostructures decreases from ∼5 to ∼3 nm. This near IR-visible PL is shown to originate from quantum confinement (QC) in Si nanostructures. Consistently, the sizes of the Si nanocrystals directly determined from HRTEM images corroborate well with those expected by QC theory.

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. 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.

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, e.g., turning the layer-by-layer growth into the Volmer-Weber growth promoted by defect-assisted nucleation. In this work, the growth of silicon on chemical vapor deposited epitaxial Gr (1 ML Gr/1 ML Gr buffer) on a 6H-SiC(0001) substrate is investigated by a combination of atomic force microscopy (AFM), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Raman spectroscopy measurements. It is shown that the perfect control of full-scale almost defect-free 1 ML Gr with a single surface structure and the ultraclean conditions for molecular beam epitaxy deposition of silicon represent key prerequisites for ensuring the growth of extended silicene sheets on epitaxial graphene. At low coverages, the deposition of Si produces large silicene sheets (some hundreds of nanometers large) attested by both AFM and SEM observations and the onset of a Raman peak at 560 cm-1, very close to the theoretical value of 570 cm-1 calculated for free-standing silicene. This vibrational mode at 560 cm-1 represents the highest ever experimentally measured value and is representative of quasi-free-standing silicene with almost no interaction with inert nonmetal substrates. From a coverage rate of 1 ML, the silicene sheets disappear at the expense of 3D Si dendritic islands whose density, size, and thickness increase with the deposited thickness. From this coverage, the Raman mode assigned to quasi-free-standing silicene totally vanishes, and the 2D flakes of silicene are no longer observed by AFM. The experimental results are in very good agreement with the results of kinetic Monte Carlo simulations that rationalize the initial flake growth in solid-state dewetting conditions, followed by the growth of ridges surrounding and eventually covering the 2D flakes. A full description of the growth mechanism is given. This study, which covers a wide range of growth parameters, challenges recent results stating the impossibility to grow silicene on a carbon inert surface and is very promising for large-scale silicene growth. It shows that silicene growth can be achieved using perfectly controlled and ultraclean deposition conditions and an almost defect-free Gr substrate.

Nanometric Moiré Stripes on the Surface of Bi2Se3 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 Bi2Se3 TI thin films and nanobelts. Scanning tunneling microscopy and high-resolution transmission electron microscopy reveal a unidirectional strained top layer, in the range 14-25%, with respect to the relaxed bulk structure, which cannot be ascribed to the mismatch with the substrate lattice but rather to strain induced by a specific growth mechanism. The 1D stripes are characterized by a spatial modulation of the local density of states, which is strongly enhanced compared to the bulk system. Density functional theory calculations confirm the experimental findings, showing that the TI surface Dirac cone is preserved in the 1D moiré stripes, as expected from the topology, though with a heavily renormalized Fermi velocity that also changes between the top and valley of the stripes. The strongly enhanced density of surface states in the TI 1D moiré superstructure can be instrumental in promoting strong correlations in the topological surface states, which can be responsible for surface magnetism and topological superconductivity.

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. These results suggest that MWCNTs are able to generate photocurrent and that the silicon substrate plays a fundamental role in our planar device. The former effect is particularly interesting because MWCNTs are generally known to mimic the electronic properties of graphite, which does not present any photocurrent generation. On the basis of theoretical calculations revealing a weak metallic character for MWCNTs, we suggest that both metallic and semiconducting nanotubes are able to generate e-h pairs upon illumination. This can be ascribed to the presence of van Hove singularities in the density of states of each single wall carbon nanotube constituting the MWCNT and to the low density of electrons at the Fermi level. Finally, we suggest that though both MWCNTs and Si substrate are involved in the photocurrent generation process, MWCNT film mainly acts as a semitransparent electrode in our silicon-based device.

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.

Ultrafast Carrier Relaxation Dynamics in Quantum Confined Non-Isotropic Silicon Nanostructures Synthesized by an Inductively Coupled Plasma Process.

To exploit the optoelectronic properties of silicon nanostructures (SiNS) in real devices, it is fundamental to study the ultrafast processes involving the photogenerated charges separation, migration and lifetime after the optical excitation. Ultrafast time-resolved optical measurements provide such information. In the present paper, we report on the relaxation dynamics of photogenerated charge-carriers in ultrafine SiNS synthesized by means of inductively-coupled-plasma process. The carriers' transient regime was characterized in high fluence regime by using a tunable pump photon energy and a broadband probe pulse with a photon energy ranging from 1.2 eV to 2.8 eV while varying the energy of the pump photons and their polarization. The SiNS consist of Si nanospheres and nanowires (NW) with a crystalline core embedded in a SiOx outer-shell. The NW inner core presents different typologies: long silicon nanowires (SiNW) characterized by a continuous core (with diameters between 2 nm and 15 nm and up to a few microns long), NW with disconnected fragments of SiNW (each fragment with a length down to a few nanometers), NW with a "chaplet-like" core and NW with core consisting of disconnected spherical Si nanocrystals. Most of these SiNS are asymmetric in shape. Our results reveal a photoabsorption (PA) channel for pump and probe parallel polarizations with a maximum around 2.6 eV, which can be associated to non-isotropic ultra-small SiNS and ascribed either to (i) electron absorption driven by the probe from some intermediate mid-gap states toward some empty state above the bottom of the conduction band or (ii) the Drude-like free-carrier presence induced by the direct-gap transition in the their band structure. Moreover, we pointed up the existence of a broadband and long-living photobleaching (PB) in the 1.2-2.0 eV energy range with a maximum intensity around 1.35 eV which could be associated to some oxygen related defect states present at the Si/SiOx interface. On the other hand, this wide spectral energy PB can be also due to both silicon oxide band-tail recombination and small Si nanostructure excitonic transition.

Gas Sensing with Solar Cells: The Case of NH3 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 NH3 in the 0-21 ppm concentration range. The PV cell parameters remarkably changed upon NH3 exposure, suggesting that these junctions, while being operated as PV cells, can react to changes in the environment, thereby acting as NH3 gas sensors. Indeed, by choosing the open-circuit voltage, VOC, 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. The sensitivity was further increased when the whole current-voltage (I-V) curve was collected and the maximum power values were tracked upon NH3 exposure.

Stoichiometric Bi2Se3 topological insulator ultra-thin films obtained through a new fabrication process for optoelectronic applications.

A new fabrication process is developed for growing Bi2Se3 topological insulators in the form of nanowires/nanobelts and ultra-thin films. It consists of two consecutive procedures: first Bi2Se3 nanowires/nanobelts are deposited by standard catalyst free vapour-solid deposition on different substrates positioned inside a quartz tube. Then, the Bi2Se3, 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. The method is new, quick, very inexpensive, easy to control and allows obtaining films with different thickness down to one quintuple layer (QL) during the same procedure. The composition and the crystal structure of both the nanowires/nanobelts and the thin films are analysed by different optical, electronic and structural techniques. For the films, scanning tunnelling spectroscopy shows that the Fermi level is positioned in the middle of the energy bandgap as a consequence of the achieved correct stoichiometry. Ultra-thin films, with thickness in the range 1-10 QLs deposited on n-doped Si substrates, show good rectifying properties suitable for their use as photodetectors in the ultra violet-visible-near infrared wavelength range.

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 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.

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 SiOx 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 SiOx 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-SiO2 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. STEM energy dispersive X-ray (EDX) spectroscopy combined with electron tomography performed on these nanostructures revealed that they contain iron, clearly demonstrating that the short ICP-synthesized SiNWs grew via an iron-catalyzed vapor-liquid-solid (VLS) mechanism within the plasma reactor. Both the STEM tomography and STEM-EDX analysis contributed to gain further insight into the self-assembly process. In the long-term, this approach might be used to optimize the synthesis of VLS-grown SiNWs via ICP as a competitive technique to the well-established bottom-up approaches used for the production of thin SiNWs.

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. To solve the growth problems, we directly deposited silicon on a chemically inert graphite substrate at room temperature. Based on atomic force microscopy, scanning tunneling microscopy, and ab initio molecular dynamics simulations, we reveal the growth of silicon nanosheets where the substrate-silicon interaction is minimized. Scanning tunneling microscopy measurements clearly display the atomically resolved unit cell and the small buckling of the silicene honeycomb structure. Similar to the carbon atoms in graphene, each of the silicon atoms has three nearest and six second nearest neighbors, thus demonstrating its dominant sp2 configuration. Our scanning tunneling spectroscopy investigations confirm the metallic character of the deposited silicene, in excellent agreement with our band structure calculations that also exhibit the presence of a Dirac cone.

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. 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.

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. Moreover, contact angle measurements show that hierarchical SWCNT/MWCNT composite surfaces exhibit a higher hydrophobicity (contact angles of up to 137°) than bare SWCNT (110°) and MWCNT (97°) coatings, thereby confirming the enhancement produced by the surface hierarchical morphology.

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. 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.

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. 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.

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. Through energy-loss spectroscopy we found that the electronic properties of these nanostructures are markedly different from those of highly oriented pyrolytic graphite (HOPG). Notably, a broadening of the π-plasmon peak in the case of MWCNTs is evident. In addition, a photocurrent was measured when MWCNTs were airbrushed onto a silicon substrate. External quantum efficiency (EQE) and photocurrent values were reported both in planar and in top-down geometry of the device. Marked differences in the line shapes and intensities were found for the two configurations, suggesting that two different mechanisms of photocurrent generation and charge collection are in operation. From this comparison, we are able to conclude that the silicon substrate plays an important role in the production of electron-hole pairs.

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. Moreover, we find that only the less oxidized nanotubes exhibit locally atomic ordered, snakeskin-like areas which possess a hexagonal arrangement which can be interpreted either as an sp(2) or sp(3) hybridized Si or Si-H layer. Their ability to not react with oxygen seems to suggest the presence of sp(2) configuration or the formation of silicon-hydrogen bonding.