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The electronic properties of graphene are highly sensitive to atoms and molecules adsorbed on its surface and to changes in its environment, such as temperature and humidity. In this paper, we examine the effect of humidity on the local carrier concentration of different types of graphene prepared by mechanical exfoliation, chemical vapour deposition and epitaxial growth on SiC, using in-situ Raman spectroscopy. We present a systematic and comparative study of the changes in Raman response using a vector analysis method to produce spatial maps of doping variation as a function of humidity. We also quantify the humidity induced carrier concentration change for different types of graphene. This study illustrates the effects of humidity on the electronic properties of graphene and provides a simple, contactless and quantitative method to directly evaluate water-induced doping effects in graphene, that are crucial for tailored device performance.

Control of atomic-scale interfaces between materials with distinct electronic structures is crucial for the design and fabrication of most electronic devices. In the case of two-dimensional materials, disparate electronic structures can be realized even within a single uniform sheet, merely by locally applying different vertical gate voltages. Here, we utilize the inherently nano-structured single layer and bilayer graphene on silicon carbide to investigate lateral electronic structure variations in an adjacent single layer of tungsten disulfide (WS2). The electronic band alignments are mapped in energy and momentum space using angle-resolved photoemission with a spatial resolution on the order of 500 nm (nanoARPES). We find that the WS2 band offsets track the work function of the underlying single layer and bilayer graphene, and we relate such changes to observed lateral patterns of exciton and trion luminescence from WS2., Sharp atomic interfaces between materials dictate the interface’s electronic properties. The authors use angle-resolved photoemission spectroscopy with a spatial resolution of ~500 nm to investigate the nanoscale electronic band structure and band alignment in a lateral heterostructure composed of WS2 placed on alternating nano-stripes of monolayer and bilayer graphene.

The effects of humidity on the electronic properties of mono- and bi-layer graphene prepared by chemical vapour deposition and transferred on SiO 2 are investigated via simultaneous global transport and local work function measurements using the van der Pauw method and Kelvin probe force microscopy, respectively. It is found that mono-layer graphene on SiO 2 is extremely sensitive to water vapour, with water molecules acting as physisorbed loosely bound p-dopants. In the case of a bi-layer stack, produced by double transfer of two graphene layers, the layers are randomly oriented and decoupled with respect to each other. As a consequence, the bottom layer of the bi-layer stack is mostly affected by substrate charges, while the top graphene layer behaves as a decoupled layer, which is externally doped by the water vapour. Moreover, we provide evidence that ambient humidity is only partly responsible for the p-doping of graphene. These findings will assist in the development of reliable graphene-based electronics such as sensors working in ambient air.

The effects of humidity on the electronic properties of quasi-free standing one layer graphene (QFS 1LG) are investigated via simultaneous magneto-transport in the van der Pauw geometry and local work function measurements in a controlled environment. QFS 1LG on 4H-SiC(0001) is obtained by hydrogen intercalation of the interfacial layer. In this system, the carrier concentration experiences a two-fold increase in sensitivity to changes in relative humidity as compared to the as-grown epitaxial graphene. This enhanced sensitivity to water is attributed to the lowering of the hydrophobicity of QFS 1LG, which results from spontaneous polarization of 4H-SiC(0001) strongly influencing the graphene. Moreover, the superior carrier mobility of the QFS 1LG system is retained even at the highest humidity. The work function maps constructed from Kelvin probe force microscopy also revealed higher sensitivity to water for 1LG compared to 2LG in both QFS 1LG and as-grown systems. These results point to a new field of applications for QFS 1LG, i.e., as humidity sensors, and the corresponding need for metrology in calibration of graphene-based sensors and devices., 16 pages, 4 figures

We study local electronic properties of graphene oxide (GO) and reduced graphene oxide (RGO) on metallic (Pt) and insulating (Si 3 N 4 ) substrates in controlled humidity environment. We demonstrate that the supporting substrate plays a crucial role in interaction of these materials with water, with Pt making both GO and RGO insensitive to humidity variations and change in environment. On the other hand, in the case of Si 3 N 4 substrate a significant difference between GO and RGO with respect to humidity variations is demonstrated, indicating complete water coverage at ∼60% R.H for RGO and ∼30% R.H. for GO. Irrespective of the substrate, both GO and RGO demonstrate relative independence of their electronic properties on the material thickness, with similar trends observed for 1 and 2 layers when subject to humidity variations. This indicates a relatively minor role of material thickness in GO-based humidity sensors.

The sensitivity to water vapour of one-, two-, and three-layer epitaxial graphene (1, 2, and 3LG) is examined in this study. It is unambiguously shown that graphene's response to water, as measured by changes in work function and carrier density, is dependent on its thickness, with 1LG being the most sensitive to water adsorption and environmental concentration changes. This is furthermore substantiated by surface adhesion measurements, which bring evidence that 1LG is less hydrophobic than 2LG. Yet, surprisingly, it is found that other contaminants commonly present in ambient air have a greater impact on graphene response than water vapor alone. This study indicates that graphene sensor design and calibration to minimize or discriminate the effect of the ambient, in which it is intended to operate, are necessary to insure the desired sensitivity and reliability of sensors. The present work will aid in developing models for realistic graphene sensors and establishing protocols for molecular sensor design and development.

The interaction of graphene oxide of varying reduction degrees with dielectric and metallic surfaces is probed in this study, in order to assess the influence that the supporting substrate has on the electronic properties of as-produced graphene oxide and its reduced form. Lateral inhomogeneities in the distribution of substrate trapped charged impurities are found to affect the electronic properties of reduced graphene oxide, giving rise to significant in-plane variations of the local electrostatic potential on reduced one-layer sheets supported on dielectric substrates. On the contrary, no such surface potential fluctuations are identified on as-produced graphene oxide sheets, or on graphene oxide layers deposited on a metallic substrate. Thicker, two-layer reduced graphene oxide sheets show effective screening of the electrostatic effects caused by charge impurities trapped in the substrate. The current study provides a useful account of the limitations that device performance could face when attempting to tune the electronic structure of graphene oxide via functionalization, highlighting the role of substrate-related disorder affecting the behaviour of nanodevices. The role of the substrate is particularly important for applications where electronic properties of graphene oxide are especially targeted, such as transparent conducting films, sensors and electrochemical devices.

The properties of epitaxial graphene on the C-face of SiC are investigated using comprehensive structural, chemical and electrical analyses. By matching similar nanoscale features on the surface potential and Raman spectroscopy maps, individual domains have been assigned to graphene patches of 1–5 monolayers thick, as well as bare SiC substrate. Furthermore, these studies revealed that the growth proceeds in an island-like fashion, consistent with the Volmer-Weber growth mode, illustrating also the presence of nucleation sites for graphene domain growth. Raman spectroscopy data shows evidence of large area crystallites (up to 620 nm) and high quality graphene on the C-face of SiC. A comprehensive chemical analysis of the sample has been provided by X-ray photoelectron spectroscopy investigations, further supporting surface potential mapping observations on the thickness of graphene layers. It is shown that for the growth conditions used in this study, 5 monolayer thick graphene does not form a continuous layer, so such thickness is not sufficient to completely cover the substrate.

Light emission in atomically thin heterostructures is known to depend on the type of materials and the number and stacking sequence of the constituent layers. Here we show that the thickness of a two-dimensional substrate can be crucial in modulating the light emission. We study the layer-dependent charge transfer in vertical heterostructures built from monolayer tungsten disulfide (WS2) on one- and two-layer epitaxial graphene, unravelling the effect that the interlayer electronic coupling has on the excitonic properties of such heterostructures. We bring evidence that the excitonic properties of WS2 can be effectively tuned by the number of supporting graphene layers. Integrating WS2 monolayers with two-layer graphene leads to a significant enhancement of the photoluminescence response, up to 1 order of magnitude higher compared to WS2 supported on one-layer graphene. Our findings highlight the importance of substrate engineering when constructing atomically thin-layered heterostructures.

Iron oxide nanowires are synthesized by photochemical method through nanosecond pulsed laser ablation of iron powder in liquid media. The synthesis is tried using various wavelengths (248 and 532 nm) of pulsed laser at different liquids (water, ethanol, methanol, isopropanol and glycol). The solution of iron oxide nanowires is obtained only in methanol when the iron powder (size < 60 μm) is irradiated at 248 nm. Ablation with 532 nm in methanol resulted in solutions of iron oxide nanoparticles. It is very interesting to note that irradiating solutions of the nanoparticles, obtained via 532 nm ablation, with laser pulses at 248 nm laser gives rise to the solution of iron oxide nanowires again. It is to be recorded that only nanoparticles are obtained in water, ethanol, isopropanol and glycol when ablated with any of the said two lasers. Chemical and structural characterization suggest that the as-synthesized products are mainly amorphous goethite [FeO(OH)]. Annealing above 400°C, converts it into crystalli...

The sensitivity of graphene to the surrounding environment is given by its {\pi} electrons, which are directly exposed to molecules in the ambient. The high sensitivity of graphene to the local environment has shown to be both advantageous but also problematic for graphene-based devices, such as transistors and sensors, where the graphene carrier concentration and mobility change due to ambient humidity variations. In this review, recent progress in understanding the effects of water on different types of graphene, grown epitaxially and quasi-free standing on SiC, by chemical vapour deposition on SiO2, as well as exfoliated flakes, are presented. It is demonstrated that water withdraws electrons from graphene, but the graphene-water interaction highly depends on the thickness, layer stacking, underlying substrate and substrate-induced doping. Moreover, we highlight the importance of clear and unambiguous description of the environmental conditions (i.e. relative humidity) whenever a routine characterisation for carrier concentration and mobility is reported (often presented as a simple figure-of-merit), as these electrical characteristics are highly dependent on the adsorbed molecules and the surrounding environment., Comment: 37 pages, 15 figures

A novel, scalable, solution-based method suitable for the control of ZnO crystal morphology at moderate temperature from rods/stars to rose-shaped, flowerlike structures is presented here. The synthesized ZnO roselike crystals are composed by wide nanosheets folded several times on themselves. This study represents the first step toward the full understanding of the growth mechanism that led to the formation of such a structure. It is now widely acknowledged that both size and shape of the metal oxide crystals have a profound effect on their properties. The influence that two morphology directing agents, ammonia and citric acid trisodium salt dihydrate, have on the shape-controlled synthesis is illustrated. The analysis of various possible mechanisms and forces that might contribute to the roselike crystals formation, such as the influence of polar surfaces, is discussed. The chemical composition and work function of the ZnO roses is investigated via XPS and UPS. XRD analysis suggests wurtzite as the most...

cited By 32; International audience; The atomic and electronic structure of a twisted and collapsed double-walled carbon nanotube was characterized using scanning tunneling microscopy and spectroscopy. It was found that the deformation opens an electronic band gap in an otherwise metallic nanotube, which has major ramifications on the use of carbon nanotubes for electronic applications. Fundamentally, the importance of the intershell interaction in this double-walled carbon nanotube points to the potential of a reversible metal-semiconductor junction, which can have device applications, as well as a caution in the design of semiconductor components based on carbon nanotubes. Lattice registry effects between the two neighboring walls evidenced by atomically resolved images confirm earlier first principle calculations indicating that the helicity influences the collapsed structure and show excellent agreement with the predicted twisted-collapse mode.

Silicon photonics is experiencing a dramatic increase in interest due to emerging application areas and several high profile successes in device and technology development (Liu et al Nature 427:615, 2004; Rong et al Nature 433:725, 2005; Almeida et al Nature 431:1081, 2004). Conventional waveguides in silicon photonics are designed for the telecom wavelengths. However, mid- and long-wave infrared regions are interesting for several application areas including sensing, communications, signal processing, missile detection and imaging (Soref et al J Opt A Pure Appl Opt 8:840, 2006). The most popular waveguide platform in silicon photonics is the Silicon-On-Insulator (SOI) structure, in the form of either a strip or a rib waveguide. This material structure, however, is not suitable for longer wavelengths (except in the 2.9–3.6 μm range) due to the absorption spectra of silicon dioxide (Soref et al J Opt A Pure Appl Opt 8:840, 2006). In this paper, we discuss the design and fabrication of two different waveguide structures, the freestanding (Yang et al Appl Phys Lett 90:241109, 2007) and hollow core waveguides (Stankovic et al Proceedings of 51th Conference ETRAN, 2007). The former is suitable for long-wave infrared applications as it has an air cladding, whilst the latter is a candidate for sensing in the mid-wave infrared wavelength region.

We investigate the local surface potential and Raman characteristics of as-grown and ex-situ hydrogen intercalated quasi-free standing graphene on 4H-SiC(0001) grown by chemical vapor deposition. Upon intercalation, transport measurements reveal a change in the carrier type from n- to p-type, accompanied by a more than three-fold increase in carrier mobility, up to μh ≈ 4540 cm2 V−1 s−1. On a local scale, Kelvin probe force microscopy provides a complete and detailed map of the surface potential distribution of graphene domains of different thicknesses. Rearrangement of graphene layers upon intercalation to (n + 1)LG, where n is the number of graphene layers (LG) before intercalation, is demonstrated. This is accompanied by a significant increase in the work function of the graphene after the H2-intercalation, which confirms the change of majority carriers from electrons to holes. Raman spectroscopy and mapping corroborate surface potential studies.

This paper reviews the process of growing large area arrays of carbon nanotubes (CNT) to be used as sensing platforms. First the work explores different methods to prepare Ni catalyst particles. More precisely, the work shows how the nanostructured arrays of catalyst are obtained by thermal annealing of Ni thin films, laser nanostructuring of Ni thin films and formation by organic dendrimers in aqueous solution. The growth process by chemical vapour deposition (CVD) is detailed and the influence of the catalyst particles on the final morphology of the tubes is discussed. A discussion of the advantages and disadvantages of each Ni nanoparticle technique to grow CNT arrays as a sensing platform is presented.

Polyaniline–fullerene (PA–C60) composites have been studied by Surface Enhanced Raman scattering (SERS) spectroscopy and X-ray diffraction. They were obtained by mixing solutions of polyaniline–emeraldine base (PA–EB) and C60, as well as by chemical synthesis from aniline, sulfuric acid and potassium dichromate, with addition of C60. The Raman bands peaking at 1330–1370 cm−1, associated with a protonated structure were used as indicative for changes of the PA–EB phonon spectrum resulting from C60 doping. The two types of compounds show different SERS spectra, also dependent on the metallic support used (Au or Ag). Variation of the SERS spectra with the type of metallic support is related to a chemical interface interaction between composite and metal. In mixture samples, a doped polymeric chain (with ionically attached C60−) was evidenced by an increased fraction of quinoid rings. The SERS spectra of the chemically synthesized PA–C60 reveals the existence of two polymeric structures: a doped PA (doped with C60− ions) and an undoped one, the latter with a structure of ‘pendant chain’ type. Structural (XRD) data reveal the presence of C60 nano-zones, with a lattice parameter increase of 0.3–0.6%, attributed to slight oxidation. Detailed analysis of the fcc(111) line asymmetry suggests, in mixture samples, a boundary zone with an ‘expanded’ lattice, induced by ionic interface effects.

The cohesive energy of the Al–Mn Mackay icosahedron (MI) was estimated for different variants of chemical order, using pair-bonding energies. This problem is related to the nucleation of i-AlMn phase in sequentially deposited thin films. The chemically ordered MI comes out as the thermodynamically most stable configuration. The role of kinetic factors during deposition is also discussed.

In common with rocksalt-type alkali halide phases and also semiconductors such as GeTe and SnTe, SnSe forms all-surface two atom-thick low dimensional crystals when encapsulated within single walled nanotubes (SWNTs) with diameters below ∼1.4 nm. Whereas previous density functional theory (DFT) studies indicate that optimised low-dimensional trigonal HgTe changes from a semi-metal to a semi-conductor, low-dimensional SnSe crystals typically undergo band-gap expansion. In slightly wider diameter SWNTs (∼1.4–1.6 nm), we observe that three atom thick low dimensional SnSe crystals undergo a previously unobserved form of a shear inversion phase change resulting in two discrete strain states in a section of curved nanotube. Under low-voltage (i.e. 80–100 kV) imaging conditions in a transmission electron microscope, encapsulated SnSe crystals undergo longitudinal and rotational oscillations, possibly as a result of the increase in the inelastic scattering cross-section of the sample at those voltages.

We present experimental studies of both local and macroscopic electrical effects in uniform single- (1LG) and bi-layer graphene (2LG) devices as well as in devices with non-uniform graphene coverage, under ambient conditions. DC transport measurements on sub-micron scale Hall bar devices were used to show a linear rise in carrier density with increasing amounts of 2LG coverage. Electrical scanning gate microscopy was used to locally top gate uniform and non-uniform devices in order to observe the effect of local electrical gating. We experimentally show a significant level of electric field screening by 2LG. We demonstrate that SGM technique is an extremely useful research tool for studies of local screening effects, which provides a complementary view on phenomena that are usually considered only within a macroscopic experimental scheme.

Capillary reactions of single-walled carbon nanotubes have been investigated both in ferrocene-saturated acetone and in ferrocene vapor. The encapsulations of ferrocene are observed as subsequent growth of inner shell tubes. From Raman spectroscopy we show that the filling yield is about ten times higher with the vapor filling. The inner tube - outer tube pair distributions are identical for " both converted samples and different to the one grown i from the fullerene precursor. The largest assigned inner shell tube from ferrocene is (11,0), which is not available [ from the fullerene precursor. This result is attributed to the catalytic growth mechanism enabling a wide diameter range of inner shell tubes. The solvent based filling provides for a quick analysis of possible molecule guest nanotube host pairs with a moderate filling factor.

from 0 s to 18 s) exhibits a stable current with a fluctuation of only 0.08 lA. Hence, the ripple measured during the extraction was not a simple noise but an oscillation that reflected a change in the interlayer structure, such as atomic configuration of carbons in neighboring layers. The observed MWCNT is in the high temperature condition, so that the electron transport in the MWCNT may be diffusive. Although further investigations are required on the influence of a thermal effect on the electrical conductivity of MWCNTs, in order to clarify the mechanism of the electrical conductivity, the measured fluctuation of the current might be a ripple corresponding to the numerical variation of p-bonding atoms in neighboring layers. Recently, it is reported that a small ripple on the sliding force during the extraction of the inner shell is obtained by the molecular mechanics calculations based on the van der Waals attraction between the interlayers, and the ripple period corresponds to that of the CNT honeycomb structure [10]. Our experimental result suggests that the ripple of the current reflects the electronic structure that orig- inated in the atomic structure of the MWCNT. In conclusion, we observed a controlled interlayer sliding deformation of MWCNTs with measuring electrical conductivity in TEM. The in situ measurement demonstrated that the electrical conductivity reversibly changed corresponding to the displacement of the inner shell. The ripple of the current might be caused by relative variation of atomic configuration in the neighboring layers. The electromechanical characteristic of MWCNTs is an attractive, as it can be utilized in developing elemental applications of nanoelectromechanical system.

The demand for high-density memory in tandem with limitations imposed by the minimum feature size of current storage devices has created a need for new materials that can store information in smaller volumes than currently possible. Successfully employed in commercial optical data storage products, phase-change materials, that can reversibly and rapidly change from an amorphous phase to a crystalline phase when subject to heating or cooling have been identified for the development of the next generation electronic memories. There are limitations to the miniaturization of these devices due to current synthesis and theoretical considerations that place a lower limit of 2 nm on the minimum bit size, below which the material does not transform in the structural phase. We show here that by using carbon nanotubes of less than 2 nm diameter as templates phase-change nanowires confined to their smallest conceivable scale are obtained. Contrary to previous experimental evidence and theoretical expectations, the nanowires are found to crystallize at this scale and display amorphous-to-crystalline phase changes, fulfilling an important prerequisite of a memory element. We show evidence for the smallest phase-change material, extending thus the size limit to explore phase-change memory devices at extreme scales.

We directly correlate the local (20-nm scale) and global electronic properties of a device containing mono-, bi- and tri-layer epitaxial graphene (EG) domains on 6H-SiC(0001) by simultaneously performing local surface potential measurements using Kelvin probe force microscopy and global transport measurements. Using well-controlled environmental conditions, where the starting state of the surface can be reproducibly defined, we investigate the doping effects of N2, O2, water vapour and NO2 at concentrations representative of the ambient air. We show that presence of O2, water vapour and NO2 leads to p-doping of all EG domains. However, the thicker layers of EG are significantly less affected by the atmospheric dopants. Furthermore, we demonstrate that the general consensus of O2 and water vapour present in ambient air providing majority of the p-doping to graphene is a common misconception. We experimentally show that even the combined effect of O2, water vapour, and NO2 at concentrations higher than typically present in the atmosphere does not fully replicate the state of the EG surface in ambient air. All doping effects can be reproducibly reversed by vacuum annealing. Thus, for EG gas sensors it is essential to consider naturally occurring environmental effects and properly separate them from those coming from targeted species., 4 Figures, 1 Table

Efforts to develop carbon nanotubes (CNTs) as nano-vehicles for precise and controlled drug and gene delivery, as well as markers for in vivo biomedical imaging, are currently hampered by uncertainties with regard to their cellular uptake, their fate in the body, and their safety. All of these processes are likely to be affected by the purity of CNT preparation, as well as the size and concentration of CNTs used, parameters that are often poorly controlled in biological experiments. It is demonstrated herein that under the experimental conditions of standard transfection methods, DWNTs are taken up by cultured cells but are then released after 24 h with no discernable stress response. The results support the potential therapeutic use of CNTs in many biomedical settings, such as cancer therapy.

By virtue of their unique electronic properties, nanometer-diameter sized single-walled carbon nanotubes represent ideal candidates to function as active parts of nanoelectronic memory storage devices. We show for the first time that GeTe, a phase change material, currently considered to be one of the most promising materials for data-storage applications, can efficiently be encapsulated within single-walled carbon nanontubes of 1.4 nm diameter. Structural investigations on the encapsulated GeTe nanowires have been carried out by high resolution transmission electron microscopy. The electronic interactions between the filling material and the host nanotube have been examined using ultraviolet photoelectron spectroscopy experiments and show that the electronic structure of the encapsulating nanotube and that of the encased filling are not perturbed by the presence of each of the other component.The newly formed hybrids offer potential to operate as active elements in non-volatile electronic memory storage devices.

We report an ultraviolet photoelectron spectroscopy study of the energetics at the interface between acid oxidised carbon nanotubes and the archetypical molecular N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'biphenyl-4,4'diamine(TPD). Electrical equilibrium is achieved across both interfaces within the experiment time frame due to the formation of an interfacial dipole layer which abruptly shifts the vacuum level at the interface. To the authors knowledge this is the first reported measurement of the electronic structure of a carbon nanotube / organic semiconductor interface; a system in which the magnitude of the dipole layer formed at the interface upon contact formation is proportional to the difference in work function between the substrate and organic semiconductor overlayer.

The electronic properties of carbon nanotubes are quite often drastically affected by the presence of defects that can develop during nanotube growth, processing or characterization too. Some of these defects such as pentagon-heptagon rings, substitutional impurities, vacancies and dislocations are of topological nature, and can sometimes create on-tube intramolecular junctions, as found by previous scanning tunnelling microscopy studies.Our recent STM experiments reveal for the first time a much more complicated junction structure, a hybrid single-walled carbon nanotube consisting of a distinct coiled structure located between two straight segments, each of different helicity. We characterise the hybrid junction at the atomic level and describe its electronic behaviour that has important implications in the practical design of functional components for nanoelectronic applications.

A new tubular form of HgTe grown in narrow single walled carbon nanotubes is described with Hg and Te in reduced coordination. Two unique projections obtained by HRTEM from two separate crystal fragments enabled reconstruction of the atomic arrangement of the new form. DFT confirmed the stability of the new structure and that it has a modified band gap, transforming HgTe from a semimetal to a semiconductor (band gap +1.3eV). HRTEM shows that as the nanotube diameter increases, the new form is no longer obtained and for diameters of 1.6-2 nm, disordered HgTe is obtained, for diameters >2 nm, sphalerite HgTe is obtained.

Single and multi junction hybrid devices comprising oligoacenes, PbS quantum dots (PbS-QD) and C60 are demonstrated. A greatly improved performance of the PbS-QD /C60 system was obtained in this work. Furthermore, a study of using acenes (pentacene, tetracene) as light absorbers and photoinduced electron donors to PbS-QD was carried out. Fabricated devices were characterized under 1 Sun simulated solar irradiation and external quantum efficiency (EQE) measurements. The initial hybrid devices demonstrated up to 0.14 % power conversion efficiency under AM1.5G illumination and up to 16 % EQE at 400 nm. The single junction (PbS-QD/C60) devices demonstrated greater than 1.2 % EQE at 1100 nm while multi junctional tetracene/PbS-QD/C60 devices exhibited up to 0.2 % EQE in the infrared region (1100 nm). Ultraviolet photoelectron spectroscopy was used to measure the molecular energy levels in the system under discussion to justify the photoinduced charge transfer. This work demonstrates new device systems which can be potentially be utilized to fabricate high efficient broad band absorbing hybrid photovoltaics.

cited By 3; International audience; Scanning Tunneling Microscopy and Spectroscopy have been used in an attempt to elucidate the electronic structure of nanotube systems containing two constituent shells. Evidence for modified electronic structure due to the inter-layer interaction in double-walled carbon nanotubes is provided by the experimental tunneling spectra and the contribution of the inner tube to the local density of states of the "composite" double-walled system is identified in agreement with previous theoretical calculations. An explicit correlation between the chirality of the two constituent tubes, the inter-wall interaction and the overall electronic structure for double-walled carbon nanotubes, is demonstrated by our experiments, showing that the effect the inner tube has on the overall electronic structure of double-walled nanotubes cannot be neglected, and is key to the opto-electronic properties of the system. We postulate that previous analysis of the opto-electronic properties on multiple-walled carbon nanotubes based purely on the outer layer chirality of the tube needs significant modification based on new understanding brought forth with our analysis. © 2008 World Scientific Publishing Company.

cited By 9; International audience; Prior to the implementation of multi-walled carbon nanotubes in microelectronic devices, investigating their electronic structure down to the nanometer scale is necessary. In that prospect, we used scanning tunneling microscopy (STM) to study the detailed atomic scale structure of double-walled carbon nanotubes, each comprising two rolled monolayers of graphene. Atomically resolved STM images usually displayed a motif and periodicity similar to that found in graphite but, on selected regions, atomically resolved motifs with a clearly defined superstructure were observed. This phenomenon has been reported previously but without a suitable explanation. We discuss the origin of this behavior in terms of modified stacking sequences due to the mismatch in registry between the chiral angles of the inner and the outer shells, associated with the interaction between the two carbon monolayers. These phenomena must be taken into account for the realization of lateral interference devices based on carbon nanotubes or graphene layers.

cited By 18; International audience; Structural and electronic properties of double-walled carbon nanotubes (DWCN) deposited on a gold substrate using both Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy (STS) were analyzed. The influence of the inner shell on the electronic structure and the transport properties of DWCNs was also discussed. The structural properties of DWCNs were determined using high resolution transmission electron microscopy (HRTEM) and atomically resolved STM. STS measurements were performed on different locations for more than 10 tubes, and the normalized differential conductance, corresponding to a spectrum of the local density of states was plotted for each tube. Experimental investigations showed that the Van Hove singularities of both the outer and the inner shell could be observed on the normalized differential conductance curves.

cited By 24; International audience; The electronic behavior of a radially collapsed armchair carbon nanotube encountered by scanning tunneling microscopy experiments is presented in a study that probes the electronic changes directly associated with the atomically resolved structural perturbations. The finite density of states obtained through scanning tunneling spectroscopy at the Fermi energy when the interspacing of the flattened faces does not allow for bond formation suggests that the electronic properties are slightly modified under mild radial deformations and provides a striking verification of previous theoretical predictions and discussions. The study clearly illustrates the challenges to be faced in the contacting of future nanowires, predicted to be the active component in integrated circuits. © 2007 The American Physical Society.

cited By 21; International audience; Evidence for modified electronic structure in double-walled carbon nanotubes with respect to their individual inner and outer constituent single-walled nanotubes is provided by scanning tunneling microscopy and spectroscopy experiments. The contribution originating from the inner tube to the local density of states of the double-walled system was identified in agreement with previous theoretical calculations. Consequently, the chiral index for the inner tube was extracted based on the additional van Hove singularities present in the experimental tunneling spectra. © 2007 American Chemical Society.

We have decoupled the intrinsic electrostatic effects arising in monolayer and few-layer MoS2 from those influenced by the flake-substrate interaction. Using ultrasonic force microscopy nanomechanical mapping, we identify the change from supported to suspended flake regions on a trenched substrate. These regions are correlated with the surface potential as measured by scanning Kelvin probe microscopy. Relative to the supported region, we observe an increase in surface potential contrast due to suppressed charge transfer for the suspended monolayer. Using Raman spectroscopy we observe a red shift of the E12g mode for monolayer MoS2 deposited on Si, consistent with a more strained MoS2 on the Si substrate compared to the Au substrate.

This paper investigates the introduction of nano-sized Ag cluster into a SiO/sub 2/ matrix by Ag/sup +/ implantation on thermally oxidised SiO/sub 2/ layers. The embedded Ag nanoclusters create conducting paths for the emissive electrons from substrate to emission surface and give rise to a local electric field enhancement due to an electrical inhomogeneity effect.

Tungsten oxide nanowires are grown directly on tungsten wires and plates using thermal heating in an acetylene and nitrogen mixture. By heating the tungsten in nitrogen ambient, single crystal tungsten oxide nanowires can be synthesized via a self-assembly mechanism. It was found that the yield can be significantly increased with the addition of acetylene, which also results in thinner nanowires, as compared to nanowires synthesized in an oxidizing ambient. The tungsten oxide nanowires are 5 to 15nm in diameter and hundreds of nanometers in length. In some cases, the use of acetylene and nitrogen process gas would result in tungsten oxide nanowires samples that appear visually transparent. Comparison of the growth using the acetylene/nitrogen or then air/nitrogen mixtures is carried out. A possible synthesis mechanism, taking into account the effect of hydrocarbon addition is proposed.

Pulse laser ablation and subsequent laser nanostructuring at room temperature has been employed to produce nanostructured Ni on SiO2/Si substrates for catalytic growth of carbon nanotubes. The resultant nanostructured surface is seen to consist of nanometer sized hemispherical droplets whose mean diameter is controlled by the initial metal thickness, which in turn is readily controlled by the number of laser pulses. Vertically aligned multiwall carbon nanotube mats were then grown using conventional plasma enhanced chemical vapor deposition. We show that within a single processing technique it is possible to produce the initial metal-on-oxide thin film to a chosen thickness but also to be able to alter the morphology of the film to desired specifications at low macroscopic temperatures using the laser parameters. The influence of the underlying oxide is also explored to explain the mechanism of nanostructuring of the Ni catalyst. (C) 2004 American Institute of Physics.

PbS nanocrystal (nc-PbS)-C60 photovoltaic devices are demonstrated, in which nc-PbS function as electron donors, showing infrared photosensitivity up to 1600nm. Annealing nc-PbS is proved to remove capping oleic acid ligands, studied using x-ray photoelectron spectroscopy, significantly improving the short circuit current, open circuit voltage, and fill factor. The device performance is rationalized by quantum confinement in nc-PbS and energy level alignment at the heterojunction based on direct measurements of nc-PbS ionization potential using ultraviolet photoelectron spectroscopy.

In this work, Ag–SiO2 nanocomposite layers were synthesized by introducing Ag nanoclusters into thermally oxidized SiO2 layers, using ion implantation. The field-emission (FE) properties of these layers were studied and correlated with the results from atomic force microscopy and transmission electron microscopy measurements. These nanocomposites exhibit good FE properties and give an emission current of 1nA at electric fields as low as 13V∕μm, for a dose of 5×1016Ag+∕cm2, compared with 204V∕μm for “bare” SiO2 layers. It is clearly demonstrated that the good FE properties of these nanocomposites are attributed to two types of local-field enhancement: one due to the surface morphology and the other due to electrical inhomogeneity. The isolated conductive Ag nanoclusters embedded in the electrically insulating SiO2 matrix provide a field enhancement due to the electrical inhomogeneity effect. Moreover, the implanted Ag ions diffuse to the surface, during the implantation process, and create dense surface-pr...

Vertically aligned carbon nanofibers (VACNFs) were synthesized on Ni-coated Si substrates using a dc plasma-enhanced chemical-vapor deposition system. The size of the Ni islands used as catalyst to grow the VACNFs was formed by both thermal annealing and laser processing on thin metal layers. It was observed that the diameter of the carbon nanofibers is strongly dependent on the initial Ni island dimension. By varying the laser power from 228 to 279mJ∕cm2, the size of these Ni islands can be controlled independent of the initial Ni film thickness. Electron field-emission results show that the emission threshold field is dependent on both the height and radius of these VACNFs and also field shielding effects. Threshold fields as low as 2V∕μm was obtained from the sample with the largest height over radius ratio.