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Synthesizing atomically thin, crystalline two-dimensional (2D) molecular materials which combine carbon with other elements is an emerging field requiring both custom-designed molecular precursors and their ability to organize into networks (hydrogen-bonded or covalent). Hybrid carbon-boron nitride (C-BN) networks face the additional challenge of needing hydrolytically-stable BN-containing molecular precursors. Here, we show that borazatruxenes (truxene-like molecules with a borazine core) and their halogenated derivatives are highly stable precursors suitable for on-surface assembly. Using scanning tunneling microscopy (STM) and density functional theory (DFT) simulations we demonstrate hierarchical H-bonded assembly based on chiral homodimers of tribromo-borazatruxenes (3Br-borazatruxenes) as building blocks for both 1D chains and 2D networks. A low-symmetry, H-bonded chiral 2D lattice forms on Au(111) from the

The density functional tight binding (DFTB) method is a fast, semi-empirical, total energy electronic structure method based upon and parameterized to density functional theory (DFT). The standard self-consistent charge (SCC) DFTB approximates the charge fluctuations in a system using a multipole expansion truncated to the monopole term. For systems with asymmetric charge distributions, such as might be induced by an applied external field, higher terms in the multipole expansion are likely to be important. We have extended the formalism to include dipoles (SCCD), have implemented the method computationally, and test it by calculating the response of various carbon nanotubes and fullerenes to an applied electric field. A comparison of polarizabilities with experimental data or more sophisticated DFT calculations indicates a substantial improvement over standard SCC-DFTB. We also discuss the issues surrounding parameterization of the new SCCD-DFTB scheme.

We demonstrate a versatile, catalyst free chemical vapor deposition process on insulating substrates capable of producing in one single stream one-dimensional (1D) WO3- x suboxides leading to a wide range of substrate-supported 2H-WS2 polymorphs: a tunable class of out-of-plane (of the substrate) nanophases, with 1D nanotubes and a pure WS2, two-dimensional (2D) nanomesh (defined as a network of webbed, micron-size, few-layer 2D sheets) at its extremes; and in-plane (parallel to the substrate) mono- and few-layer 2D domains. This entails a two-stage approach in which the 2WO3 + 7S → 2WS2 + 3SO2 reaction is intentionally decoupled. First, various morphologies of nanowires or nanorods of high stoichiometry, WO2.92/WO2.9 suboxides (belonging to the class of Magnéli phases) were formed, followed by their sulfurization to undergo reduction to the aforementioned WS2 polymorphs. The continuous transition of WS2 from nanotubes to the out-of-plane 2D nanomesh, via intermediary, mixed 1D-2D phases, delivers tunable functional properties, for example, linear and nonlinear optical properties, such as reflectivity (linked to optical excitations in the material), and second harmonic generation (SHG) and onset of saturable absorption. The SHG effect is very strong across the entire tunable class of WS2 nanomaterials, weakest in nanotubes, and strongest in the 2D nanomesh. Furthermore, a mechanism via suboxide (WO3- x) intermediate as a possible path to 2D domain growth is demonstrated. 2D, in-plane WS2 domains grow via "self-seeding and feeding" where short WO2.92/WO2.9 nanorods provide both the nucleation sites and the precursor feedstock. Understanding the reaction path (here, in the W-O-S space) is an emerging approach toward controlling the nucleation, growth, and morphology of 2D domains and films of transition-metal dichalcogenides.

We demonstrate a versatile, catalyst free chemical vapor deposition process on insulating substrates capable of producing in one single stream one-dimensional (1D) WO3–x suboxides leading to a wide range of substrate-supported 2H-WS2 polymorphs: a tunable class of out-of-plane (of the substrate) nanophases, with 1D nanotubes and a pure WS2, two-dimensional (2D) nanomesh (defined as a network of webbed, micron-size, few-layer 2D sheets) at its extremes; and in-plane (parallel to the substrate) mono- and few-layer 2D domains. This entails a two-stage approach in which the 2WO3 + 7S → 2WS2 + 3SO2 reaction is intentionally decoupled. First, various morphologies of nanowires or nanorods of high stoichiometry, WO2.92/WO2.9 suboxides (belonging to the class of Magnéli phases) were formed, followed by their sulfurization to undergo reduction to the aforementioned WS2 polymorphs. The continuous transition of WS2 from nanotubes to the out-of-plane 2D nanomesh, via intermediary, mixed 1D-2D phases, delivers tunable functional properties, for example, linear and nonlinear optical properties, such as reflectivity (linked to optical excitations in the material), and second harmonic generation (SHG) and onset of saturable absorption. The SHG effect is very strong across the entire tunable class of WS2 nanomaterials, weakest in nanotubes, and strongest in the 2D nanomesh. Furthermore, a mechanism via suboxide (WO3–x) intermediate as a possible path to 2D domain growth is demonstrated. 2D, in-plane WS2 domains grow via “self-seeding and feeding” where short WO2.92/WO2.9 nanorods provide both the nucleation sites and the precursor feedstock. Understanding the reaction path (here, in the W–O–S space) is an emerging approach toward controlling the nucleation, growth, and morphology of 2D domains and films of transition-metal dichalcogenides.

© 2016 American Chemical Society. Developing generic strategies for building adaptable or multifunctional bioplatforms is challenging, in particular because protein immobilization onto surfaces often causes loss of protein function and because multifunctionality usually necessitates specific combinations of heterogeneous elements. Here, we introduce a generic, modular bioplatform construction strategy that uses cage-like supramolecular multienzyme complexes as highly adaptable building blocks immobilized directly and noncovalently on graphene. Thermoplasma acidophilum dihydrolipoyl acyltransferase (E2) supramolecular complexes organize as a monolayer or can be controllably transferred onto graphene, preserving their supramolecular form with specific molecular recognition capability and capacity for engineering multifunctionality. This E2-graphene platform can bind enzymes (here, E1, E2's physiological partner) without loss of enzyme function; in this test case, E1 catalytic activity was detected on E2-graphene over 6 orders of magnitude in substrate concentration. The E2-graphene platform can be multiplexed via patterned cotransfer of differently modified E2 complexes. As the E2 complexes are robust and highly customizable, E2-graphene is a platform onto which multiple functionalities can be built.

Currently, there is no available needle-free approach for diabetics to monitor glucose levels in the interstitial fluid. Here, we report a path-selective, non-invasive, transdermal glucose monitoring system based on a miniaturized pixel array platform (realized either by graphene-based thin-film technology, or screen-printing). The system samples glucose from the interstitial fluid via electroosmotic extraction through individual, privileged, follicular pathways in the skin, accessible via the pixels of the array. A proof of principle using mammalian skin ex vivo is demonstrated for specific and ‘quantized’ glucose extraction/detection via follicular pathways, and across the hypo- to hyper-glycaemic range in humans. Furthermore, the quantification of follicular and non-follicular glucose extraction fluxes is clearly shown. In vivo continuous monitoring of interstitial fluid-borne glucose with the pixel array was able to track blood sugar in healthy human subjects. This approach paves the way to clinically relevant glucose detection in diabetics without the need for invasive, finger-stick blood sampling.

We have successfully coated CNTs with either positively or negatively charged polyelectrolytes employing Van der Waals interactions between nanotubes and polyelectrolyte molecules, without any previous chemical modification of the nanotubes walls. Polymer adsorption of polyelectrolytes was performed in an ethanolic solution, followed by redispersion in water. The surface coating was verified with ζ-potential measurements and SEM. A sequential assembly of polyelectrolyte layers on top of the polyelectrolyte modified nanotubes has been achieved from a water solution following the layer-by-layer technique. The assembly of polyelectrolytes from water on CNTs deposited on a surface has also been shown with QCM-D. Then, polyelectrolyte coated CNTs have been used as positive and negative blocks in the templating of layer-by-layer films.

Crystalline silver nanowires 60-100 nm in diameter and tens of micrometres in length have been fabricated using a low temperature, solution synthesis technique. We explore the potential of this method to produce functional nanowire structures using two different strategies to attach active molecules to the nanowires: adsorption and displacement. Initially, as-produced silver nanowires capped with a uniaxial-growth-inducing polymer layer were functionalised by solution adsorption of a semiconducting conjugated polymer to generate fluorescent nanowire structures. The influence of nanowire surface chemistry was investigated by displacing the capping polymer with an alkanethiol self-assembled monolayer, followed by solution adsorption functionalisation. The success of molecular attachment was monitored by electron microscopy, absorption and fluorescence spectroscopy and confocal fluorescence microscopy. We examined how the optical properties of such adsorbed molecules are affected by the metallic nanowires, and observed transfer of excitation energy between dye molecules mediated by surface plasmons propagating on the nanowires. Non-contact dynamic force microscopy measurements were used to map the work-function of individual wires, revealing inhomogeneity of the polymer surface coverage.

Thermal stability and reactivity to oxidation of several nanocomposite systems obtained by encapsulation of metal halides in single-walled carbon nanotubes are studied. Thermogravimetric analysis coupled with Raman spectroscopy allows insight into the various contributing factors, such as charge transfer, strain, and defect formation, and establishing a hierarchy of reactivity for the systems studied (AgX@SWCNTs, with X = Br, I; SWCNTs = arc discharge and HiPCO). The activation energy for oxidation decreases considerably after filling, indicating that filled nanotubes are more amenable to controlled modifications based on chemical reactivity than the originating empty nanotubes. The complete removal of the carbon shell at high temperatures does not preserve the nanowire morphology of the encapsulated halides; these are freed on surfaces in the form of nanoparticles arranged in 1D patterns. Metallic nanoparticles were obtained after hydrogen reduction of the halides, and growth of silicon nanowires in the footprint of the originating nanocomposites was demonstrated from such Co seeds. MX@SWCNTs (M = Ag, Co) can thus be used as environmentally stable nanoscale containers that allow the deliverance of catalytic nanoparticles in a prepatterned and aligned way.

Rhenium diselenide (ReSe2) is a layered indirect gap semiconductor for which micromechanical cleavage can produce monolayers consisting of a plane of rhenium atoms with selenium atoms above and below. ReSe2 is unusual among the transition-metal dichalcogenides in having a low symmetry; it is triclinic, with four formula units per unit cell, and has the bulk space group P1̅. Experimental studies of Raman scattering in monolayer, few-layer, and bulk ReSe2 show a rich spectrum consisting of up to 16 of the 18 expected lines with good signal strength, pronounced in-plane anisotropy of the intensities, and no evidence of degradation of the sample during typical measurements. No changes in the frequencies of the Raman bands with layer thickness down to one monolayer are observed, but significant changes in relative intensity of the bands allow the determination of crystal orientation and of monolayer regions. Supporting theory includes calculations of the electronic band structure and Brillouin zone center phonon modes of bulk and monolayer ReSe2 as well as the Raman tensors determining the scattering intensity of each mode. It is found that, as for other transition-metal dichalcogenides, Raman scattering provides a powerful diagnostic tool for studying layer thickness and also layer orientation in few-layer ReSe2.

It is shown that sp2 phase organization plays an important role in the field emission from nanostructured carbons. Emission is found to depend on the cluster size, anisotropy, and mesoscale bonding of the sp2 phase, and the electronic disorder. It is found by Raman spectroscopy that increasing the size of sp2 clusters in the 1–10 nm range improves emission. Anisotropy in the sp2 phase orientation can help or inhibit the emission. sp2 clusters embedded in the sp3 matrix or electronic disorder induced by localized defects oriented in the field direction can provide a local field enhancement to facilitate the emission.

Field emission has been reported to occur at much lower fields in carbon based thin film systems than from any other material systems. The emission has been shown to depend on the various material parameters, but whichever carbon based system is used, it is found that emission occurs at localised sites rather than uniformly over the entire surface. Carbon films with mixed sp 3 /sp 2 bonding, like nanocrystalline diamond and nanocluster graphitic films emit at lower fields with a higher emission site density than single-phase films. The sp 2 cluster size in any carbon film can be altered during deposition, but it is easier to control nanocluster size by post-deposition annealing. Annealing increases the sp 2 cluster size embedded in a sp 3 matrix until the sp 3 matrix disappears completely and the film transforms into nanocrystalline graphite. To distinguish the effects of the sp 2 cluster size from other material parameters, a series of different carbon films were annealed post-deposition and the sp 2 cluster size was measured using visible Raman. Field emission was then measured at a vacuum of 10 −8 mbar on all films using a parallel plate configuration. It was found that the field emission for all films tested depended upon the clustering of the sp 2 phase and this effect dominates the effects of the other parameters, such as chemical composition, surface termination, sp 3 content or conductivity. The optimum size of the sp 2 was of the order of 1 nm for all systems tested. We believe that field emission occurs form the localised conducting, predominantly sp 2 bonded regions, which provideds the large field enhancement required for effective emission.

Electronic transport and photoconductivity in amorphous carbons have been studied in order to understand their transport, recombination and photoluminescence properties. The role of the density of states and the effect of sp 2 phase nanostructuring are discussed. Localisation effects due to hydrogen bonding are shown.

The work function of tetrahedral amorphous carbon (ta-C) has been measured by Kelvin probe to lie in the range 4–5 eV, irrespective of its sp3 content or nitrogen addition. This implies that the surface barrier to emission is dominant and that emission changes caused by sp3 bonding or nitrogen addition are not directly due to changes in work function. Hydrogen, oxygen, and argon plasma treatments are all found to increase the emission of a-C, but hydrogen and argon treatments are found to reduce the work function while oxygen treatment increases it. Detailed studies of the surface with varying plasma treatment conditions suggest that the changes in emission arise mainly from changes in the surface microstructure, such as the formation of sp2 regions within the sp3 bulk. The need for local field enhancement mechanisms to account for emission over the sizeable barrier is emphasized, which may arise from local chemical nonhomogeneity, or formation of nanometer-size sp2 clusters embedded in an sp3 matrix.

Junction capacitance measurements and voltage-pulse stimulated capacitance transient measurements have been applied to undoped hydrogenated tetrahedral amorphous carbon (ta-C:H)/crystalline-Si (c-Si) heterostructures to deduce the defect densities in as-grown and post-annealed ta-C:H films. Transient capacitance measurements reveal a component corresponding to the emission of carriers out of defect states at the ta-C:H/c-Si interface, and a slower component corresponding to the carrier emission from defect states of interior ta-C:H. The interior defect density of as-grown films was estimated at 1×1018±2×1017cm−3, decreasing to (6.5±2)×1017cm−3 for films annealed at 300°C. A Gaussian defect band was identified with activated hole emission with a 0.35 eV activation energy. Steady state admittance measurements indicate two thermal activation processes and have allowed us to determine a defect density of 1×109±2×108cm−2 at the ta-C:H/c-Si interface.

The photoconductivities of various sp3 bonded amorphous carbons were studied to understand their recombination and photoluminescence processes. At temperatures >200 K, the photoconductivity of each type of carbon does not vary inversely with the paramagnetic defect density, indicating that paramagnetic defects are not the only recombination centres. This effect occurs because carrier demarcation levels lie in the band tails, so both defects and tail states act as recombination centres. The temperature regime

The changes in the photoconductivity of hydrogenated tetrahedral amorphous carbon (ta-C:H) with nitrogen incorporation were studied. Low level nitrogen incorporation improves the photoconductivity, by shifting the Fermi level upwards in the band gap. Films with a photosensitivity of about 200 at room temperature under white light illumination of 35 mW/cm2 were obtained; thus is the highest value so far reported for diamond-like carbons. At high temperatures, photoconductivity is controlled by nonradiative recombination through gap states, whereas at low temperatures it occurs by energy-loss hopping in the band tails. Nitrogen addition does not create extra charge defect recombination centers. Low temperature photoconductivity allows the direct determination of the localization radius of the band tail states. This radius varies from 2–3 A in ta-C:H to 9 A in ta-C. This illustrates how hydrogen can increase state localization and the photoluminescence efficiency in amorphous carbons.

The steady-state photoconductivity of tetrahedral amorphous carbon (ta-C) and hydrogenated ta-C (ta-C:H) has been studied as a function of temperature, light intensity, and photon energy, in order to understand the transport and recombination process in diamond-like carbon. It is found that the levels demarking the recombination states can span only part of the gap, so that the recombination centres can vary from every defect, to some defects, to some tail states, according to conditions.

The photoconductivity of tetrahedral amorphous carbon (ta-C) and hydrogenated tetrahedral amorphous carbon (ta-C:H) has been studied as a function of temperature, photon energy, and light intensity in order to understand the transport and recombination processes. ta-C and ta-C:H are found to be low mobility solids with μτ products of order 10−11–10−12 cm2/V at room temperature because of their relatively high defect densities. Deep defects tend to be the dominant recombination centers, but at high and moderate temperatures only a fraction of these centers or even tail states can act as recombination centers because the carrier demarcation levels do not always span the gap. For excitation by high energy UV photons, a peak in the photoconductivity is found at 200 K, similar to the thermal quenching effect found in a-Si:H, and attributed to competitive recombination between two classes of centers with very different capture cross sections.

The study of the electric field dependence of carrier thermal emission from deep traps has provided important information on deep defects in III-V semiconductors. In this paper we analyze the effect of the electric field on the electron-hole thermal generation process in hydrogenated amorphous silicon. A detailed model is proposed to explain the experimental field dependence of the thermal generation process. The calculation was carried out by taking into account the trivalent statistics for deep states and two field mechanisms, which enhance the carrier emission, the Poole-Frenkel effect, and thermally assisted tunneling. The Poole-Frenkel effect alone is not strong enough to account for the experimental field dependence of the thermally generated current, and the thermally assisted tunneling mechanism must be included in order to reproduce the experimental results. If a strong electron-phonon coupling is considered within an ``electron-lattice interaction'' model for the thermally assisted tunneling process, a good agreement with the experimental data is obtained for a carrier effective mass value ${m}^{*}{=0.3m}_{e}.$ This value is very close to that predicted by effective-mass theory calculations for amorphous silicon. We discuss the possibility of including in the model other nonstandard processes, such as the lowering of the mobility edge induced by the electric field and defect relaxation. Hierarchical defect relaxation processes may explain some of our previous results concerning high electric field forming of reverse biased $p\ensuremath{-}i\ensuremath{-}n$ diodes.

We present the first study of quantum interference patterns at a bilayer-trilayer armchair interface, for different stacking sequences. Visualization using scanning tunneling microscopy and theoretical calculations provides direct evidence that near armchair edges electron behavior is dominated by the “hard” edge, where the layer is abruptly truncated, as opposed to the “soft” edges, where layers continue across the boundary. Intervalley reflection causes universal quenching of the wavefunction with a periodicity of three C atoms, while the exact interference patterns depend on the stacking sequence and appear to be robust to disorder and chemical terminations.

The leakage current of thick (up to 20 μm) a-Si:H p-i-n diodes developed for nuclear radiation detection were investigated. Performing current-voltage measurements at different temperatures and over a large electric field domain (up to 106 V/cm), different leakage current mechanisms, such as electron injection and field enhanced thermal generation, have been identified. The influence of the p layer thickness on the electronic injection was clearly demonstrated by means of spectral response experiments. The effect of a device forming procedure on the leakage current and on the noise power spectra of the devices has also been studied.

20 /spl mu/m thick p-i-n diodes were deposited at high rate using a PECVD process. The p-i-n structures have been specifically tailored in order to sustain high reverse bias voltages and exhibit low leakage currents. Mean electric fields up to 5.10/sup 5/ V/cm have been applied to our devices. We report the electrical characterization of such devices in terms of I(V,T) curves. Different regimes have been identified and the activation energy of the reverse current is deduced. A spectral analysis of the noise has been performed. Electronic transport of both electrons and holes has been studied using a time of flight technique. Finally, the detector response to alpha and beta particles, and X-rays has been studied in order to measure the collection efficiencies of thick a-Si:H detectors.

A metastable evolution of a-Si:H p-i-n diodes submitted to a high reverse bias for a long period of time was observed. This forming process leads to a significant decrease of the reverse current and increase of the breakdown voltage. The mechanisms underlying this forming process have been investigated using the constant photocurrent method experiment. The effects of annealing and current-induced defect creation have been studied. The forming process is found to be most consistent with a metastable increase in the dopant activation, leading to an improvement of the p-layer blocking efficiency.

Repair is ubiquitous in biological systems, but rare in the inorganic world. We show that inorganic nanoscale systems can however possess remarkable repair and reconfiguring capabilities when subjected to extreme confinement. Confined crystallization inside single-walled carbon nanotube (SWCNT) templates is known to produce the narrowest inorganic nanowires, but little is known about the potential for repair of such nanowires once crystallized, and what can drive it. Here inorganic nanowires encapsulated within SWCNTs were seen by high-resolution transmission electron microscopy to adjust to changes in their nanotube template through atomic rearrangement at room temperature. These observations highlight nanowire repair processes, supported by theoretical modeling, that are consistent with atomic migration at fractured, ionic ends of the nanowires encouraged by long-range force fields, as well as release-blocking mechanisms where nanowire atoms bind to nanotube walls to stabilize the ruptured nanotube and allow the nanowire to reform. Such principles can inform the design of nanoscale systems with enhanced resilience. © 2012 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

We describe measurements of the stiffness and Young’s modulus, Y of single crystals of molybdenum-based compounds. Atomic force microscopy is used first to image, and then to perform stiffness measurements on crystals which are growing up out of a substrate. Y is extracted by comparing the measured stiffness with that calculated from a continuum elasticity model of the crystals, whose experimentally indeterminate parameters are the Young’s modulus and the geometry. We find a value for Y in the range 0.8–1.4 TPa, depending on the assumed geometry of the nanocrystal. As these crystals are essentially perfectly ordered on the micron scale, this opens the possibility of forming composite materials of immense strength.

The large variety of hybrid carbon nanotube systems synthesized to date (e.g., by encapsulation, wrapping, or stacking) has provided a body of interactions with which to modify the host nanotubes to produce new functionalities and control their behavior. Each, however, has limitations: hybridization can strongly degrade desirable nanotube properties; noncovalent interactions with molecular systems are generally weak; and interlayer interactions in layered nanotubes are strongly dependent upon the precise stacking sequence. Here we show that the electrostatic/polarization interaction provides a generic route to designing unprecedented, sizable and highly modulated (1 eV range), noncovalent on-tube potentials via encapsulation of inorganic partially ionic phases where charge anisotropy is maximized. Focusing on silver iodide (AgI) nanowires inside single-walled carbon nanotubes, we exploit the polymorphism of AgI, which creates a variety of different charge distributions and, consequently, interactions of varying strength and symmetry. Combined ab initio calculations, high-resolution transmission electron microscopy, and scanning tunneling microscopy and spectroscopy are used to demonstrate symmetry breaking of the nanotube wave functions and novel electronic superstructure formation, which we then correlate with the modulated, noncovalent electrostatic/polarization potentials from the AgI filling. These on-tube potentials are markedly stronger than those due to other noncovalent interactions known in carbon nanotube systems and lead to significant redistribution of the wave function around the nanotube, with implications for conceptually new single-nanotube electronic devices and molecular assembly. Principles derived can translate more broadly to relating graphene systems, for designing/controlling potentials and superstructures.

Electron field emission from amorphous carbon is found to depend on the clustering of the sp2 phase. The size of the sp2 phase is varied by thermal annealing and it dominates the effect of other parameters, such as chemical composition, surface termination, sp3 content, or conductivity. The optimum size of the sp2 phase is determined by Raman spectroscopy and is of the order of 1 nm. The field emission originates from the sp2 regions and is facilitated by the large field enhancement from more conductive sp2 clusters in an insulating sp3 matrix.

Junction capacitance measurements performed on tetrahedrally bonded amorphous carbon (ta-C)/crystalline-Si (c-Si) heterostructures are used to deduce the defect density in thin ta-C films. Using the drive level capacitance profiling technique, we determine a homogeneous defect density of 6±1.5×1017 cm−3 in the undoped (p-type) ta-C films and one order of magnitude larger in the nitrogen-doped (n-type) films. Admittance measurements reveal defect states at the ta-C/c-Si interface with a density exceeding 2×1012 cm−2.

We succeeded in integrating individual, pre-existing nanostructures into functional devices using ultrahigh vacuum dynamic nanostenciling and show working devices based on single-walled carbon nanotubes, a benchmark nanomaterial, and porphyrin J-aggregates, a "soft" supramolecular nanomaterial. Nanostructures are first located via atomic force microscopy, while device elements are added step by step, with an achieved positional accuracy of 20 nm, using a shadow mask assembly that moves while being exposed to evaporated material. Electronic transport, potentiometry, and scanning Kelvin probe were used for control at any fabrication stage and were available in situ. Such complex fabrication/characterization capabilities, applicable repeatedly, reliably, and nondestructively, pave the way for dynamic nanostenciling instrumentation to establish itself as a viable tool for easy integration and prototyping of fragile nanostructures synthesized through a wide range of processes.

The effect of KI encapsulation in narrow (HiPCO) single-walled carbon nanotubes is studied via Raman spectroscopy and optical absorption. The analysis of the data explores the interplay between strain and structural modifications, bond-length changes, charge transfer, and electronic density of states. KI encapsulation appears to be consistent with both charge transfer and strain that shrink both the C-C bonds and the overall nanotube along the axial direction. The charge transfer in larger semiconducting nanotubes is low and comparable with some cases of electrochemical doping, while optical transitions between pairs of singularities of the density of states are quenched for narrow metallic nanotubes. Stronger changes in the density of states occur in some energy ranges and are attributed to polarization van der Waals interactions caused by the ionic encapsulate. Unlike doping with other species, such as atoms and small molecules, encapsulation of inorganic compounds via the molten-phase route provides stable effects due to maximal occupation of the nanotube inner space.

This paper is concerned with the characterization of new detectors fabricated from a-Si:H films deposited at high rates through the dilution of SiH/sub 4/ in Helium by PECVD (plasma enhanced chemical vapor deposition) technique. Rates up to ten times (5.5 /spl mu/m/h) that of the standard technique are obtained. We have investigated the electrical characteristics -depletion voltage, residual space charge density- of the helium diluted material and compared them to that of the standard material. Finally, the response of detectors, fabricated from both materials, to 5.5 MeV alpha particles are compared. >

A comprehensive instrument, designed for fabricating nanostructures by evaporation through a dynamic shadow mask in ultrahigh vacuum, is described. The versatility and performance of the instrument is demonstrated through a series of examples, allowing for applications that are impossible to achieve with traditional nanopatterning methods. Clean nanostructures or entire devices made of different materials and on various substrates can be fabricated. The technique is compatible with fundamental surface science and can be easily interfaced with other fabrication and characterization techniques.

UHV dynamic force and energy dissipation spectroscopy in non-contact atomic force microscopy were used to probe specific interactions with composite systems formed by encapsulating inorganic compounds inside single-walled carbon nanotubes. It is found that forces due to nano-scale van der Waals interaction can be made to decrease by combining an Ag core and a carbon nanotube shell in the Ag@SWNT system. This specific behaviour was attributed to a significantly different effective dielectric function compared to the individual constituents, evaluated using a simple core-shell optical model. Energy dissipation measurements showed that by filling dissipation increases, explained here by softening of C-C bonds resulting in a more deformable nanotube cage. Thus, filled and unfilled nanotubes can be discriminated based on force and dissipation measurements. These findings have two different implications for potential applications: tuning the effective optical properties and tuning the interaction force for molecular absorption by appropriately choosing the filling with respect to the nanotube., Comment: 22 pages, 6 figures

The possibility of achieving nanometer-scale sensitivity in a surface-enhanced Raman (SERS) experiment while using larger-sized probes (0.1--1 \ensuremath{\mu}m) is investigated. The application targeted is carbon film transformation under high-energy beam irradiation, and, primarily, the transformation of amorphous carbon into nanocrystalline graphite. The carbon film covers nanometer-size Ag particles which enhance the signal from zones of material adjacent to them. This geometry gives access to the film/substrate interface, and in this way it complements scanning near-field techniques which have similar spatial sensitivity but are mainly surface sensitive. The SERS effect has been studied as a function of the Ag nanoparticle size, carbon film thickness, and excitation wavelength. A selective enhancement of the Raman cross section of the D band of amorphous carbons was observed. The dielectric properties of the carbon film, when used as an overlayer, strongly affect the SERS enhancement, so that changes in the dielectric function upon irradiation can be used to produce local enhancement contrast, and to establish an identification procedure for material transformation.

We have applied junction capacitance and transient photocapacitance measurements to undoped tetrahedral amorphous carbon (ta-C)/silicon carbide (SiC) heterostructures to deduce defect densities and defect distributions in ta-C. The junction capacitance measurements show two thermally activated processes. One can be related to the activation of carriers out of defects at the ta-C/SiC interface while the other one with an activation energy of 0.36eV is an intrinsic property of the ta-C. The defect density at the ta-C/SiC interface is estimated to be roughly 9 ± 2 × 109 cm−2. The transient photocapacitance measurements have allowed us to observe the broader band tail of ta-C, giving a value (Urbach energy) of 230meV.

Tetrahedrally bonded amorphous carbon (ta-C) is a predominately sp3-bonded semiconductor with a band gap of order 2 eV. It can be doped n-type using nitrogen but no successful p-type doping has been reported until now. On the other hand, it has recently been shown that the incorporation of boron reduces the intrinsic compressive stress of ta-C, while still maintaining its high fraction of sp3 sites. This paper reports a detailed study of the electrical properties of boron-doped ta-C (ta-C:B). The ta-C:B films are deposited in a filtered cathodic vacuum arc system using a pressed cathode of graphite and boron powder. The composition and structure of the films are examined by electron energy loss spectroscopy. We find that the room temperature conductivity of the films increases by five orders of magnitude with a boron concentration from 0 to 8%. The conductivity activation energy decreases for the same boron concentration, while the E04 gap remains constant. N-type silicon/ta-C:B heterojunctions show a rectifying behaviour as a function of the boron concentration of the films. The films show photo-conductivity. The combined results indicate p-type doping of ta-C.

We report the results of junction capacitance measurements on thin tetrahedral amorphous carbon (ta-C) films to deduce their defect densities. We find defect densities in the range 3 - 7 × 1017 cm−3 in the undoped ta-C films, and roughly an order of magnitude larger in the nitrogen doped (n-type) films. In some cases fairly uniform defect profiles were obtained covering a thickness of a couple of hundreds angstroems. We also observed a thermal activation process of carriers from defect states at the ta-C/c-Si interface with an activation energy in the range of 0.4eV to 0.5eV.

Electronic applications of diamond-like carbon have been limited by its relatively high disorder and defect density. We find that the density of paramagnetic defects in hydrogenated tetrahedral amorphous carbon and the Urbach slope of the optical absorption edge can be reduced by annealing at 300 °C, with little effect on the optical gap. This leads to a reduction in the dark conductivity and an increase in the photosensitivity. The effect is attributed to the migration of hydrogen through the C–C network, to allow better passivation of dangling bonds and a modification of the more weakly bonded sp2 clusters with narrower local band gaps.

We have studied the electric field dependence of the electron-hole thermal generation process in hydrogenated amorphous silicon. A model was developed which takes into account the Poole-Frenkel effect and the thermally assisted tunneling. In order to explain the experimental results it was necessary to consider a strong electron-lattice interaction describing the carrier tunneling mechanism. Deep defects relaxation is also discussed.

Amorphous hydrogenated p-i-n diodes submitted to a high reverse bias for a long period of time undergo a metastable evolution which tends to improve their properties. A forming procedure based on this effect has been developed and leads to a significant decrease of the reverse current and increase of the breakdown voltage. In this paper, the mechanisms underlying this forming process have been investigated using the Constant Photocurrent Method (CPM), (I-V) characteristics and Spectral Response under reverse bias. The effects of annealing and current induced defect creation have been studied. The forming process is found to be most consistent with a metastable increase in the dopant activation of the p-layer and a small decrease of the density of states in the i-layer.

We used electrostatic force and Kelvin probe microscopies to investigate few-layer graphene (FLG) domains on top of ionic crystals. Step edges, pits, and protrusions within the ionic surface create sizeable and local perturbations of the surface potential of graphene overlayers. These were within the eV range in FLG with up to three layers, and become considerably screened in thicker layers. Engineering such nanostructures in a regular manner can allow the bottom-up creation of on-sheet p–n junctions and superlattices that exploit the Dirac nature of carriers in graphene, and provide a test bed for studying local screening.

PECVD hydrogenated Amorphous silicon (a-Si:H) layers have been deposited at high rates using helium dilution of the reacting precursors. This allows the fabrication of very thick layers (a few tens of microns) within reasonable processing times. This technique can be applied to the fabrication of thick p-i-n detector devices to be used for direct X-ray detection. Several p-i-n devices have been processed at deposition rates as high as 15 A/sec without significant changes in the optoelectronic properties of the a-Si:H films. Capacitance-Voltage measurements have been performed in order to measure the residual space charge density in the intrinsic region of the devices. The results demonstrate a moderate increase of the N*DoS concentration with the deposition rate. DC bias photoconductivity measurements have also been performed. The experimental data have been fitted with a multiple trapping transport model for holes. The model parameters have been studied and related to the electronic properties of the films.

We describe a strategy for fabricating devices on perforated silicon nitride membranes that is ideal for systems with modulated structure, in particular for nanotubes with endohedral inorganic compounds or molecules, or nanotubes externally functionalized with soft materials. It uses dynamic nanostenciling and focussed ion beam while avoiding nanotubes’ exposure to chemicals or beams. Physical properties and high resolution transmission electron microscopy are then correlated. Devices with individual single-walled carbon nanotubes partially filled with potassium iodide (KI) showed negative differential resistance (NDR). We attribute the NDR to electrostatic potential modulation on the nanotube due to permanent dipoles in the KI chain.