Professor David Carey

Electron paramagnetic resonance measurements have been made on samples of float zone silicon, implanted with 10^15 Er/cm2. One sample was coimplanted with oxygen to give an impurity concentration of 10^20 O/cm3 and 10^19 Er/cm3. In this coimplanted sample, sharp lines are observed which are identified as arising from a single spin 1/2 Er3+ center having a g tensor exhibiting monoclinic C1h symmetry. The principal g values and tilt angle are g1=0.80, g2=5.45, g3=12.60, and τ=2.6°. In the absence of O, the sharp lines are not observed. No Er3+ cubic centers were detected in either sample. Possible structures for the center are discussed

The electrical properties of amorphous carbon are governed by the high localization of the sp2 π states, and conventional methods of altering the sp2 content result in macroscopic graphitization. By using ion beams we have achieved a delocalization of the π states by introducing nanoclustering and hence improving the connectivity between existing clusters, as demonstrated by the increase in the conductivity by two orders of magnitude without modification of the band gap. At higher doses, paramagnetic relaxation-time measurements indicate that exchange effects are present. This unveils the possibility of amorphous carbon-based electronics by tailoring the ion-beam conditions, which we demonstrate in the form of a rectifying device.

Three new [Ru(bpy)2X]+ complex ions, where bpy represents bipyridyl ligand and X denotes pyridyl diazolate or pyrazinyl diazolate coordination site, have been computationally designed and synthesized as pH-sensitive molecules. The choice of pyridyl and pyrazinyl moieties allows for the nitrogen content to vary, whereas the influence of the protonation site is quantified by using 1,2-diazolate and 1,3-diazolate derivatives. The absorption and emission properties of the deprotonated and protonated complex ions were characterized by UV–vis and photoluminescence spectroscopy as well as by time-dependent density functional theory. Protonation causes (1) a strong blue shift in the lowest energy 3MLCT → S0 emission wavelengths, (2) a substantial increase in the emission intensity, and (3) a change in the character of the corresponding 3MLCT emitting states. The blue shift in the emission wavelength becomes less pronounced when the nitrogen content in the X-ligand increases and when going from 1,2- to 1,3-diazolate derivatives. The contrast in the emission intensity of the protonated/deprotonated forms is the highest for the complex ion, containing a 2-pyridyl derivative of the 1,2-diazolate. The complex ions are suggested as potential pH-responsive materials based on change in the color and intensity of the emitted radiation. The broad impact of the research demonstrates that the modification of the nitrogen content and position within the protonable ligands is an effective approach for modulation of the pH-optosensing properties of Ru–polypyridyl complexes.

Ultrasonication is widely used to exfoliate two dimensional (2D) van der Waals layered materials such as graphene. Its fundamental mechanism, inertial cavitation, is poorly understood and often ignored in ultrasonication strategies resulting in low exfoliation rates, low material yields and wide flake size distributions, making the graphene dispersions produced by ultrasonication less economically viable. Here we report that few-layer graphene yields of up to 18% in three hours can be achieved by optimising inertial cavitation dose during ultrasonication. We demonstrate that inertial cavitation preferentially exfoliates larger flakes and that the graphene exfoliation rate and flake dimensions are strongly correlated with, and therefore can be controlled by, inertial cavitation dose. Furthermore, inertial cavitation is shown to preferentially exfoliate larger graphene flakes which causes the exfoliation rate to decrease as a function of sonication time. This study demonstrates that measurement and control of inertial cavitation is critical in optimising the high yield sonication-assisted aqueous liquid phase exfoliation of size-selected nanomaterials. Future development of this method should lead to the development of high volume flow cell production of 2D van der Waals layered nanomaterials.

The ability to induce an energy band gap in bilayer graphene is an important development in graphene science and opens up potential applications in electronics and photonics. Here we report the emergence of permanent electronic and optical band gaps in bilayer graphene upon adsorption of π electron containing molecules. Adsorption of n- or p-type dopant molecules on one layer results in an asymmetric charge distribution between the top and bottom layers and in the formation of an energy gap. The resultant band gap scales linearly with induced carrier density though a slight asymmetry is found between n-type dopants, where the band gap varies as 47 meV/10(13) cm(-2), and p-type dopants where it varies as 40 meV/10(13) cm(-2). Decamethylcobaltocene (DMC, n-type) and 3,6-difluoro-2,5,7,7,8,8-hexacyano-quinodimethane (F2-HCNQ, p-type) are found to be the best molecules at inducing the largest electronic band gaps up to 0.15 eV. Optical adsorption transitions in the 2.8-4 μm region of the spectrum can result between states that are not Pauli blocked. Comparison is made between the band gaps calculated from adsorbate-induced electric fields and from average displacement fields found in dual gate bilayer graphene devices. A key advantage of using molecular adsorption with π electron containing molecules is that the high binding energy can induce a permanent band gap and open up possible uses of bilayer graphene in mid-infrared photonic or electronic device applications.

The quantification of disorder and the effects of clustering in the sp2 phase of amorphous carbon thin films are discussed. The sp2 phase is described in terms of disordered nanometer sized conductive sp2 clusters embedded in a less conductive sp3 matrix. Quantification of the clustering of the sp2 phase is estimated from optical as well as from electron and nuclear magnetic resonance methods. Unlike in other disordered group IV thin film semiconductors, we show that care must be exercised in attributing a meaning to the Urbach energy extracted from absorption measurements in the disordered carbon system. The influence of structural disorder, associated with sp2 clusters of similar size, and topological disorder due to undistorted clusters of different sizes is also discussed. Extensions of this description to other systems are also presented.

Pulsed-laser (248 nm) irradiation of Ag thin films was employed to produce nanostructured AgSi O2 substrates. By tailoring the laser fluence, it was possible to controllably adjust the mean diameter of the resultant near-spherical Ag droplets. Thin films of tetrahedral amorphous carbon (ta-C) were subsequently deposited onto the nanostructured substrates. Visible Raman measurements were performed on the ta-C films, where it was observed that the intensity of the Raman signal was increased by nearly two orders of magnitude, when compared with ta-C films grown on nonstructured substrates. The use of laser annealing as a method of preparing substrates, at low macroscopic temperatures, for surface-enhanced Raman spectroscopy on subnanometer-thick films is discussed. © 2006 American Institute of Physics.

Carbon-based materials exhibit distinct structures and dimensionality which allow modification of their electrical properties and enable them to be integrated in various commercial systems. One of the interesting characteristics of carbon-based materials is efficient electron field emission (FE), which makes them good candidates for displays, in electron microscopy, lithography, sensing, micro- and nanoelectronics, X-ray sources and medical applications. While nano carbon materials have been extensively studied for FE applications, their usefulness, electron emission concerns, and fundamental mechanisms for FE technologies are buried in the reported literature, and cross comparison of all nano carbon materials together is rarely explored. Here we present a comprehensive overview of fundamental and FE properties of all carbon-based materials including diamond, nanocrystalline diamond, graphite, carbon nanotubes, carbon nanofibers, carbon nanowires, carbon nanorods, graphene, and amorphous and nanostructured carbon. Some of these carbon materials, such as amorphous and nanostructured carbon, possess the added benefit of room temperature production over large areas on a variety of substrates. We have compiled an up to date summary which critically discusses the material factors, and the factors that control electron emission of these materials. We also propose unique ideas to further improve electron emission for the design of energy efficient carbon-based cold cathode materials for next generation large area electronic devices.

We present a performance comparison of polythiophene/fullerene derivative bulk heterojunction solar cells fabricated on fluorinated tin oxide (FTO) and indium tin oxide (ITO) in the presence and absence of the commonly used poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) hole extraction layer. From a potential commercial perspective the performance of cheaper and more readily available FTO compares well with the more expensive ITO in terms of measured device efficiency (FTO:2.8 % and ITO:3.1%). The devices show similar fill factors (FTO:63% and ITO:64%) with the same open circuit voltage of 0.6 V. The short circuit current density is lower for FTO devices at 7.5 mA/cm2 which compares with 8.0 mA/cm2 for ITO; a behaviour that is mainly attributed to the reduced optical transmission of the FTO layer. Importantly, these devices were part fabricated and wholly characterized under atmospheric conditions. The quoted device performance is the best reported for FTO based bulk heterojunction systems in the absence of the highly acidic PEDOT:PSS hole extraction layer, which is believed to degrade conductive oxides.

Carbon nanotube (CNT)–silicon (Si) heterojunctions show exceptional electrical behavior and hence are promising for electronic and optoelectronic applications. In particular, single wall CNTs (SWCNTs)–Si heterojunctions have been widely studied for these applications. Since multiwall CNTs (MWCNTs) have higher electrical conductivity than SWCNTs, engineering the properties of MWCNTs so as to tailor their electrical properties suitable for heterojunctions can boost the performance of CNT‐based electronic and optoelectronic devices. Here the development of MWCNT‐Si heterostructures is reported, following surface functionalization and silanization to tailor their structure and properties, at room temperature via solution processing. The developed Al/n‐Si/MWCNT/Al heterojunction devices show a low turn‐on voltage (≈1–3 V) and high current (≈0.8 mA at 10 V) exceeding the previous high temperature processed CNT‐based heterojunctions as well as room temperature grown additional amorphous carbon–Si heterojunctions. The carrier transport mechanism within a carrier‐selective contact, multijunction, multiresistance framework, with device current–voltage behavior dictated by transport across the heterojunction and quantum tunneling is discussed. This work opens new direction to design improved devices for future development of large area solution processable CNT based electronics. Multiwall carbon nanotube–silicon (MWCNT–Si) heterostructures, following surface functionalization and silanization to tailor their structure and properties, are developed at room temperature via solution processing. The developed Al/n‐Si/MWCNT/Al heterojunction devices show a low turn‐on voltage and high current exceeding the previous high temperature processed CNT‐based heterojunctions as well as room temperature grown additional amorphous carbon–Si heterojunctions.

The transfer of an electron from a carbon nanotube (CNT) tip into vacuum under a high electric field is considered beyond the usual one-dimensional semi-classical approach. A model of the potential energy outside the CNT cap is proposed in order to show the importance of the intrinsic CNT parameters such as radius, length and vacuum barrier height. This model also takes into account set-up parameters such as the shape of the anode and the anode-to-cathode distance, which are generically portable to any modelling study of electron emission from a tip emitter. Results obtained within our model compare well to experimental data. Moreover, in contrast to the usual one-dimensional Wentzel–Kramers–Brillouin description, our model retains the ability to explain non-standard features of the process of electron field emission from CNTs that arise as a result of the quantum behaviour of electrons on the surface of the CNT.

Erbium implanted silicon as a quantum technology platform has both telecommunications and integrated circuit processing compatibility. In Si implanted with Er to a concentration of 3 × 1017 cm−3 and O to a concentration of 1020 cm−3, the electron spin coherence time, T2, and the spin-lattice relaxation time, T1, were measured to be 7.5 μs and ∼1 ms, respectively, at 5 K. The spin echo decay profile displayed strong modulation, which was consistent with the super-hyperfine interaction between Er3+ and a spin bath of 29Si nuclei. The calculated spectral diffusion time was similar to the measured T2, which indicated that T2 was limited by spectral diffusion due to T1-induced flips of neighboring Er3+ spins. The origin of the echo is an Er center surrounded by six O atoms with monoclinic C1h site symmetry.

The observation of electron emission from amorphous carbon thin films at low applied electric fields is explained in terms of an enhancement of the field brought about by dielectric inhomogeneities within the film. These inhomogeneities originate from the differences between conductive, spatially localized sp2 C clusters surrounded by a more insulating sp3 matrix. By a more complete understanding of the concentration and distribution of the clusters, a generic model for field emission from amorphous carbon thin films can be developed. Extensions of this model to explain the emission properties of carbon nanotubes and carbon nanocomposite materials are also presented.

The field-emission-display (FED) technology examined in the early sixties used metal tips or Spindt cathodes in order to extract electron beams to excite phosphors. The tips were necessitated by the large work function the electrons needed to overcome in order to be released into the vacuum. In the early nineties it was noticed that 'flat' diamond surfaces emitted electrons at relatively low electric fields. Just as its crystalline counterparts, amorphous-carbon thin films also showed that this class of materials were also capable of electron emission at low threshold fields. By using flat emitters, technologist can remove a number of fabrication steps that otherwise would have been required to produce large-area arrays of field emitters and therefore reduce the cost of production significantly. This paper will review the progress of the use of flat amorphous semiconductors as cold cathodes. New results that appear to point towards a space-charge-controlled emission mechanism as opposed to a purely surface emission process based upon Fowler-Nordheim tunneling will be introduced, which have implications on the type of device structure that will ultimately be needed for electron field-emission devices. Two possible cold-cathode materials, namely, amorphous-carbon and amorphous silicon, will be examined.

The influence of the concentration and size of sp2 carbon clusters on the field emission properties of hydrogenated amorphous carbon thin films is investigated. In combination with electron paramagnetic resonance and optical measurements, it is shown that the trend in the threshold field for emission for films deposited under certain conditions can be explained in terms of improvements in the connectivity between sp2 clusters. These clusters are believed to be located near the Fermi level, and the connectivity is primarily determined by the cluster size and concentration, which in turn is determined by the choice of deposition conditions. Details of the appropriate emission mechanisms for different types of deposited carbon films are discussed.

The growth of graphene on Ni using a photo-thermal chemical vapor deposition (PT-CVD) technique is reported. The non-thermal equilibrium nature of PT-CVD process resulted in a much shorter duration in both heating up and cooling down stages, thus allowing for a reduction in the overall growth time. Despite the reduced time for synthesis compared to standard thermal chemical vapor deposition (T-CVD), there was no decrease in the quality of the graphene film produced. Furthermore, the graphene formation under PT-CVD is much less sensitive to cooling rate than that observed for T-CVD process. Growth on Ni also allows for the alleviation of hydrogen blister damage that is commonly encountered during growth on Cu substrates and a lower processing temperature. To characterize the film’s electrical and optical properties, we further report the use of pristine PT-CVD grown graphene as the transparent electrode material in an organic photovoltaic device (OPV) with poly(3-hexyl)thiophene (P3HT)/phenyl-C61-butyric acid methyl ester (PCBM) as the active layer where the power conversion efficiency of the OPV cell is found to be comparable to that reported using pristine graphene prepared by conventional CVD.

The identification of new materials capable of sustaining a high electron emission current is a key requirement in the development of the next generation of cold cathode devices and technology. Compatibility with large volume material production methods is a further important practical consideration with solution chemistry-based methods providing for route to industrial scale-up. Here we demonstrate a new class of organic-inorganic hybrid material based on polypyrrole and zinc oxide (PPy/ZnO) nanofibers for use as a low-cost large-area cathode material. Solution chemistry based surfactant chemical oxidation polymerisation is used to synthesise the nanofibers and the macroscopic turn-on electric field for emission has been measured to be as low as 1.8 V/μm, with an emission current density of 1 mA/cm2 possible for an applied electric field of less than 4 V/μm. Specfic surface area measurements reveal a linear increase in the nanofiber surface area with ZnO incorporation, which when coupled with electron microscopy and x-ray diffraction analysis reveals that the wurtzite ZnO nanoparticles (around 45 nm in size) act as nucleation sites for the growth of PPy nanofibers. Our study demonstrates for the first time how an inorganic nanocrystal acting as a nucleation site allows for the tailored growth of the organic component without diminishing the overall electrical properties and opens the potential of a new type of organic-inorganic hybrid large-area cathode material. The broader impacts and advantages of using hybrid materials, when compared to other composite nanomaterial systems, as large area cathode materials are also discussed

This paper presents a fully-transparent and novel frequency selective surface (FSS) that can be deployed instead of conventional glass to reduce the penetration loss encountered by millimeter wave (mmWave) frequencies in typical outdoorindoor (O2I) communication scenarios. The presented design uses a 0:035 mm thick layer of indium tin oxide (ITO), which is a transparent conducting oxide (TCO) deposited on the surface of the glass, thereby ensuring the transparency of the structure. The paper also presents a novel unit cell that has been used to design the hexagonal lattice of the FSS structure. The dispersion and transmission characteristics of the proposed design are presented and compared with conventional glass. The presented FSS can be used for both 26 GHz and 28 GHz bands of the mmWave spectrum and offers a lower transmission loss as compared to conventional glass without any considerable impact on the aesthetics of the building infrastructure.

Cluster-assembled nanostructured carbon with a fractal morphology is employed as a large-area surface scaffold for metal decoration. By depositing silver by pulsed laser ablation densely packed, distributions of metal nanoparticles are produced. The authors show, using the surface-enhanced Raman effect and the modification of fluorescence quantum yields near metallic surfaces, that silver-coated nanostructured carbon can be used to sense low concentrations of biomolecules.

We have investigated the room temperature long channel field effect characteristics of a single graphene layer transistor incorporating a poly-4-vinyl-phenol (PVP) organic insulating layer, as an alternative to conventional oxide gate dielectric materials. High purity copper foils were used in the chemical vapour growth of the graphene layer and visible Raman analysis confirmed the presence of a high quality mono-layer carbon film. Using a channel length of 50 μm, a field effect hole mobility of 37 cm2/Vs was calculated, which demonstrates the possibility of an all carbon graphene based large area transistor with carrier mobilities above those found in conventional long channel all organic electronic transistors.

This article reports on the formation and electronic characteristics of conducting carbon nanowires produced by swift heavy ion irradiation of a fullerene thin film. This study shows that it is possible to create arrays of carbon nanowires, which are perfectly parallel to each other and perpendicular to the substrate. As-deposited fullerene films exhibit poor field emission characteristics with breakdown fields as high as 51 V/mu m, whereas low dose ion irradiated fullerene film produces a threshold field as low as 9 V/mu m. The present approach of making conducting carbon nanowires by ion irradiation for potential field emitters and large area applications is also discussed.

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.

Ruddlesden-Popper phase (RPP) perovskites of the form A1n−1A22BnX3n+1 show great promise in stable photovoltaic (PV) devices or as light emitting diodes (LEDs). In particular, n= 1, mono-layer RPPs of the form ABX4 have also shown great progress as the passivating layer for 3D perovskite PVs. We study the electronic behaviour of mixed B site An−1Pb1-xSnxX3n+1 where A = PEA or MA to investigate if the size of the A site cation indirectly affects the nonlinear band gap dependence of a 2D monolayer RPP layer. Both perovskites show a nonlinear behaviour primarily due to the relative energy difference between the Sn 5s - I 5p antibonding states and the Pb 6s - I 5p antibonding states, though the extent of the nonlinearity is reduced relative to 3D bulk perovskites due to the reduced dimensionality of these 2D structures. We also discuss the influence on bandgap nonlinearity due to the structural distortions induced by the differences between the A site cation. This research presents a strategy to the design of mixed solid state 2D perovskites by tuning the structural parameters as well as metal and halide composition.

The effects of electrical current stressing on the field emission characteristics of hydrogenated amorphous carbon (a-C:H) thin films are reported. In these a-C:H films an initial conditioning treatment of the film is often required before the onset of stable emission and only after several voltage cycles do the values of the threshold field tend to converge. By stressing of the film by applying a predetermined current through the film, the initial conditioning treatment can be removed and stable and reproducible emission observed. Retesting of the current stressed films shows that the films remain fully conditioned provided a sufficiently high stress current was initially used.

In this study, arc discharge multiwall carbon nanotubes have been incorporated into PmPV polymer, a derivative of the conjugated polymer PPV. Electron field emission characterisation was made as a function of nanotube mass fraction using large area phosphor coated anode. It is concluded that emission occurs at front surface under the action of applied field. As the mass fraction of nanotubes is increased, a greater density of nanotube-nanotube contacts is made, increasing the overall charge transport of carriers via FIT. Replenishment of the electrons occurring by a percolation controlled charge transport through the disordered nanotube network.

Electron paramagnetic resonance measurements have been made on samples of float zone silicon, implanted with 10^15 Er/cm2. One sample was coimplanted with oxygen to give an impurity concentration of 10^20 O/cm3 and 10^19 Er/cm3. In this coimplanted sample, sharp lines are observed which are identified as arising from a single spin 1/2 Er3+ center having a g tensor exhibiting monoclinic C1h symmetry. The principal g values and tilt angle are g1=0.80, g2=5.45, g3=12.60, and τ=2.6°. In the absence of O, the sharp lines are not observed. No Er3+ cubic centers were detected in either sample. Possible structures for the center are discussed

Pulsed-laser (248 nm) irradiation of Ag thin films was employed to produce nanostructured Ag/SiO2 substrates. By tailoring the laser fluence, it was possible to controllably adjust the mean diameter of the resultant near-spherical Ag droplets. Thin films of tetrahedral amorphous carbon (ta-C) were subsequently deposited onto the nanostructured substrates. Visible Raman measurements were performed on the ta-C films, where it was observed that the intensity of the Raman signal was increased by nearly two orders of magnitude, when compared with ta-C films grown on nonstructured substrates. The use of laser annealing as a method of preparing substrates, at low macroscopic temperatures, for surface-enhanced Raman spectroscopy on subnanometer-thick films is discussed.

Effects of ion implantation on electron centers were investigated in hydrogenated amorphous carbon films. Electron spin resonance and Raman spectra measurements were carried out during the analysis. It was found that ion implantation reduces the contents of hydrogen and initiates the radiation defects in hydrogenated amorphous carbon films.

The observation of field induced electron emission from room temperature grown carbon nanofibers at low (5 V/mum) macroscopic electric fields is reported. The nanofibers were deposited using methane as a source gas in a conventional rf plasma enhanced chemical vapor deposition reactor using a Ni metal catalyst previously subjected to an Ar plasma treatment. Analysis of the scanning electron microscopy images of the nanofibers show them to possess an average diameter of 300 nm and that the nanofibers are observed to be radially dispersed over an area of 50 mum in diameter. No evidence of hysteresis in the current-voltage characteristic or conditioning of the emitters is observed. The mechanism for emission at low fields is attributed to field enhancement at the tips rather than from the surrounding amorphous carbon film which is shown to have a higher threshold field (20 V/mum) for emission.

Computational 3D simulations of individual carbon nanotubes (CNTs) and vertically aligned arrays of CNTs were conducted in order to gain insight into their field emission characteristics. We find that the widely used approximation that the enhancement factor is equal to the aspect ratio of the emitter to be an over-estimation, and that the enhancement factor is only loosely related to the aspect ratio, among other parameters.

"The reactive ion etching of quartz and Pyrex substrates was carried out using CF4/Ar and CF4/O2 gas mixtures in a combined radio frequency (rf)/microwave (µw) plasma. It was observed that the etch rate and the surface morphology of the etched regions depended on the gas mixture (CF4/Ar or CF4/O2), the relative concentration of CF4 in the gas mixture, the rf power (and the associated self-induced bias) and microwave power. An etch rate of 95 nm/min for quartz was achieved. For samples covered with a thin metal layer, ex situ high resolution scanning electron microscopy and atomic force microscopy imaging indicated that, during etching, surface roughness is produced on the surface beneath the thin metallic mask. Near vertical sidewalls with a taper angle greater than 80° and smooth etched surfaces at the nanometric scale were fabricated by carefully controlling the etching parameters and the masking technique. A simulation of the electrostatic field distribution was carried out to understand the etching process using these masks for the fabrication of high definition features.

Previous theoretical and experimental work has shown that surface tension gradients in liquid layers create surface defects and inhibit the levelling of an uneven surface. In coatings deposited from thermosetting polyester powders, which are studied here, small amounts of a low molecular-weight acrylate are incorporated to act as a “flow agent.” We find that this additive lowers the surface tension of the polymer melt and has a minor effect on the melt viscosity. A slower rate of levelling results from the decreased surface tension. We provide experimental evidence that lateral gradients in the surface tension of the polymer melt, resulting from the non-uniform distribution of the flow agent, inhibit the levelling of the surface. Specifically, the surface roughness of a powder coating is up to three times greater when a steep surface tension gradient is purposely created through powder blending. Surface tension gradients might also be responsible for the greater surface roughness (observed with atomic force microscopy on lateral length scales of 100 μm) that is found in coatings that contain flow agent.

Excimer laser irradiation is used to crystallize hydrogenated amorphous silicon thin films. The resulting films show a stratified microstructure with a crystalline volume fraction of up to 90%. There is a range of excimer laser energy that can produce stratified nanocrystalline silicon with a Tauc gap as high as 2.2 eV. This value is greater than that of amorphous or crystalline silicon and is contrary to that predicted from the theoretical analysis of mixed-phase silicon thin films. The phenomenon is explained by employing transmission electron microscopy and spectroscopic ellipsometry, and the observed bandgap enhancement is associated with quantum confinement effects within the nanocrystalline silicon layers, rather than an impurity variation.

Visible room-temperature photoluminescence (PL) was observed from hydrogen-free nanostructured amorphous carbon films deposited by pulsed laser ablation in different background pressures of argon (P-Ar). By varying P-Ar from 5 to 340 mTorr, the film morphology changed from smooth to rough and at the highest pressures, low-density filamentary growth was observed. Over the same pressure regime an increase in the ordering of sp(2) bonded C content was observed using visible Raman spectroscopy. The origin of the PL is discussed in terms of improved carrier localization within an increased sp(2) rich phase.

Erbium implanted silicon is promising for both photonic and quantum technology platforms, since it possesses both telecommunications and integrated circuit processing compatibility. However, several different Er centres are generated during the implantation and annealing process, the presence of which could hinder the development of these applications. When Si is co-implanted with 1017 cm-3 Er and 1020 cm-3 O ions, and the appropriate annealing process is used, one of these centres, which is present at higher Er concentrations, can be eliminated. Characterisation of samples with Er concentrations

The nanostructure of amorphous carbon thin films is described in terms of a disordered nanometer-sized conductive sp(2) phase embedded in an electrically insulating sp(3) matrix. It is shown that the degree of clustering and disorder within the sp(2) phase plays a determining role in the electronic properties of these films. Clustering of the sp(2) phase is shown to be important in explaining several experimental results including the reduction of the electron spin resonance linewidth with increasing spin density and the dispersion associated with the width of the Raman active G band. The influence of structural disorder, associated with sp(2) clusters of similar size, and topological disorder, due to undistorted clusters of different sizes, on both spin density and Raman measurements, is discussed. An extension of this description to intercluster interactions to explain some of the electrical transport and electron field emission behavior is also presented.

For carbon nanotubes (CNTs) to be exploited in electronic applications, the growth of high quality material on conductive substrates at low temperatures (

Future generation local communication systems will need to employ THz frequency bands capable of transferring sizable amounts of data. Current THz technology via electrical excitation is limited by the upper limits of device cutoff frequencies and by the lower limits of optical transitions in quantum confined structures. Current metallic THz antennas require high power to overcome scattering losses and tend to have low antenna efficiency. We show here via calculation and simulation that graphene can sustain electromagnetic propagation at THz frequencies via engineering the intra- and interband contributions to the dynamical conductivity to produce a variable surface impedance microstrip antenna with a several hundred GHz bandwidth. We report the optimization of a circular graphene microstrip patch antenna on silicon with an optimized return loss of -26 dB, a -10 dB bandwidth of 504 GHz and an antenna efficiency of -3.4 dB operating at a frequency of 2 THz. An improved antenna efficiency of -0.36 dB can be found at 3.5 THz but is accompanied by a lower bandwidth of about 200 GHz. Such large bandwidths and antenna efficiencies offers significant hope for graphene based flexible directional antennas that can be employed for future THz local device-to-device communications.

The electrical properties of amorphous carbon are governed by the high localization of the sp π states, and conventional methods of altering the sp content result in macroscopic graphitization. By using ion beams we have achieved a delocalization of the π states by introducing nanoclustering and hence improving the connectivity between existing clusters, as demonstrated by the increase in the conductivity by two orders of magnitude without modification of the band gap. At higher doses, paramagnetic relaxation-time measurements indicate that exchange effects are present. This unveils the possibility of amorphous carbon-based electronics by tailoring the ion-beam conditions, which we demonstrate in the form of a rectifying device.

Er implanted Si is a candidate for quantum and photonic applications; however, several different Er centres are generated, and their symmetry, energy level structure, magnetic and optical properties, and mutual interactions have been poorly understood, which has been a major barrier to the development of these applications. Optically modulated magnetic resonance (OMMR) gives a spectrum of the modulation of an electron paramagnetic resonance (EPR) signal by a tuneable optical field. Our OMMR spectrum of Er implanted Si agrees with three independent measurements, showing that we have made the first measurement of the crystal field splitting of the 4I13/2 manifold of Er implanted Si, and allows us to revise the crystal field splitting of the 4I15/2 manifold. This splitting originates from a photoluminescence (PL) active O coordinated Er centre with orthorhombic C2v symmetry, which neighbours an EPR active O coordinated Er centre with monoclinic C1h symmetry. This pair of centres could form the basis of a controlled NOT (CNOT) gate.

The field induced emission from room temperature grown carbon nanofibers at low macroscopic electric field was discussed. It was found that the nanofiber were deposited using methane as a source gas in a conventional rf plasma enhanced chemical vapor deposition reactor. It was observed that nanofibers possessed an average diameter of 300 nm. Analysis shows that the mechanism for emission at low fields was attributed to field enhancement at the tips.

he mechanisms controlling the nanostructuring of thin metal-on-oxide films by nanosecond pulsed excimer lasers are investigated. When permitted by the interfacial energetics, the breakup of the metal film into nanoscale islands is observed. A range of metals (Au, Ag, Mo, Ni, Ti, and Zn) with differing physical and thermodynamic properties, and differing tendencies for oxide formation, are investigated. The nature of the interfacial metal-substrate interaction, the thermal conductivity of the substrate, and the initial thickness of the metal film are all shown to be of importance when discussing the mechanism for nanoscale island formation under high fluence irradiation. It is postulated that the resulting nanoparticle size distribution is influenced by the surface roughness of the initial film and the Rayleigh instability criterion. The results obtained are compared with simulations of the heat transfer through the film in order to further elucidate the mechanisms. The results are expected to be applicable to the laser induced melting of a large range of different materials, where poor wetting of substrate by the liquid phase is observed.

The electronic properties of disordered carbon based materials can be discussed in terms of the clustering of the sp2 carbon phase and delocalization of the electron wavefunction. In smooth amorphous carbon thin films this results in a mixed phase material of conductive sp2 clusters embedded in an electrically insulating sp3 matrix. The delocalization of the electron wavefunction associated with the sp2 clusters is shown to play an important role in understanding many of the electronic and optical properties of the films. It is demonstrated that the extent of the electron delocalization and clustering can be estimated using magnetic resonance methods. Evidence for delocalization in a range of carbon based materials such as diamond-like carbon thin films produced by chemical vapour deposition, nanostructured carbon produced by pulsed laser ablation and ultrananocrystalline diamond is presented.

A simple approach is proposed for obtaining low threshold field electron emission from large area diamond-like carbon (DLC) thin films by sandwiching either Ag dots or a thin Ag layer between DLC and nitrogen-containing DLC films. The introduction of silver and nitrogen is found to reduce the threshold field for emission to under 6 V/μm representing a near 46% reduction when compared with unmodified films. The reduction in the threshold field is correlated with the morphology, microstructure, interface, and bonding environment of the films. We find modifications to the structure of the DLC films through promotion of metal-induced sp bonding and the introduction of surface asperities, which significantly reduce the value of the threshold field. This can lead to the next-generation, large-area simple and inexpensive field emission devices. © 2013 American Chemical Society.

In situ three terminal electron field emission characterization of an isolated multiwalled carbon nanotube has been performed, where both anode and gate electrodes are attached to high precision piezodrivers. All measurements are performed in a scanning electron microscope allowing accurate knowledge of the local environment of the nanotube to be obtained. It is shown that the presence of the grounded gate electrode screens the applied field by approximately 32%. This technique in positioning the gate and anode electrodes allows for an estimate of the gate transparency factor and demonstrates characterization of individual carbon nanotubes without the need for fabrication of arrays of emitters.

The attempt to integrate two electron transport mechanisms, by computing the field emission (FE) current as a superposition of both resonant and sequential branches, is reported. A quantum object through which the tunnelling occurs is modelled as a one-dimensional (1D) potential well separated from the substrate by a potential barrier. Results show that the proposed model is useful in the interpretation of real I-V diagrams of FE through composite vacuum interfaces and in evaluating the amount of coherence in the related tunnelling processes.

The effects of electrical current stressing on the field emission characteristics of hydrogenated amorphous carbon (a-C:H) thin films are reported. In these a-C:H films an initial conditioning treatment of the film is often required before the onset of stable emission and only after several voltage cycles do the values of the threshold field tend to converge. By stressing of the film by applying a predetermined current through the film, the initial conditioning treatment can be removed and stable and reproducible emission observed. Retesting of the current stressed films shows that the films remain fully conditioned provided a sufficiently high stress current was initially used.

This article reports on the formation and electronic characteristics of conducting carbon nanowires produced by swift heavy ion irradiation of a fullerene thin film. This study shows that it is possible to create arrays of carbon nanowires, which are perfectly parallel to each other and perpendicular to the substrate. As-deposited fullerene films exhibit poor field emission characteristics with breakdown fields as high as 51 V/μm, whereas low dose ion irradiated fullerene film produces a threshold field as low as 9 V/μm. The present approach of making conducting carbon nanowires by ion irradiation for potential field emitters and large area applications is also discussed.

Considerable effort is currently expounded on the development and improvement of the myriad display technologies that have come to the market place. In this paper, several key questions are addressed in the development of the future generation of large-area field-emission-based displays based on semiconducting amorphous carbon thin films and carbon nanotubes (CNTs). The development of carbon-based cathodes has to date proceeded along empirical lines, with attempts to correlate the variation of field-emission characteristics with changes in deposition or post-deposition processing parameters, often without a full explanation being forthcoming. In addition, there have been incidents of incorrect interpretation of some of the results, due to a lack of appreciation of the significant differences between the different types of amorphous carbon film that exist. It is only recently that a fuller understanding of the different electron-emission mechanisms has begun to emerge through an understanding of the roles played by the electrical and structural inhomogeneity at nanometre level. This 'intrinsic dielectric inhomogeneity' is shown to possess some remarkable electronic properties, which also have important consequences for extrinsic inhomogeneous nanometre systems such as CNT and CNT-polymer-composite-based displays. The future outlook for broad-area displays based on amorphous carbon and CNTs is also addressed.

The observation of field induced electron emission from room temperature grown carbon nanofibers at low (5 V/µm) macroscopic electric fields is reported. The nanofibers were deposited using methane as a source gas in a conventional rf plasma enhanced chemical vapor deposition reactor using a Ni metal catalyst previously subjected to an Ar plasma treatment. Analysis of the scanning electron microscopy images of the nanofibers show them to possess an average diameter of 300 nm and that the nanofibers are observed to be radially dispersed over an area of 50 µm in diameter. No evidence of hysteresis in the current-voltage characteristic or conditioning of the emitters is observed. The mechanism for emission at low fields is attributed to field enhancement at the tips rather than from the surrounding amorphous carbon film which is shown to have a higher threshold field (20 V/µm) for emission.

The observation of electron emission from amorphous carbon thin films at low applied electric fields is explained in terms of an enhancement of the field brought about by dielectric inhomogeneities within the film. These inhomogeneities originate from the differences between conductive, spatially localized sp2 C clusters surrounded by a more insulating sp3 matrix. By a more complete understanding of the concentration and distribution of the clusters, a generic model for field emission from amorphous carbon thin films can be developed. Extensions of this model to explain the emission properties of carbon nanotubes and carbon nanocomposite materials are also presented.

The nanostructure of amorphous carbon thin films is described in terms of a disordered nanometer-sized conductive sp2 phase embedded in an electrically insulating sp3 matrix. It is shown that the degree of clustering and disorder within the sp2 phase plays a determining role in the electronic properties of these films. Clustering of the sp2 phase is shown to be important in explaining several experimental results including the reduction of the electron spin resonance linewidth with increasing spin density and the dispersion associated with the width of the Raman active G band. The influence of structural disorder, associated with sp2 clusters of similar size, and topological disorder, due to undistorted clusters of different sizes, on both spin density and Raman measurements, is discussed. An extension of this description to intercluster interactions to explain some of the electrical transport and electron field emission behavior is also presented.

An enhancement of the electron emission current from carbon nanotubes (CNTs) can be achieved by increasing the local electric field or reduction of the potential barrier to emission. Chemical modification of the surface of CNTs is often used to achieve good dispersion but the effects on the field emission properties are often overlooked. We demonstrate that significant improvements in the current density of CNT-based arrays can be achieved through chemically induced changes to the nanotube work function. For a given current density the requirements for the local electric field and work function are calculated.

Sequential two-step electron tunneling from nanoparticles has been studied from a transport point of view. A steady-state balance equation for the partial electron currents is derived and solved to obtain the stationary level occupancy, which leads to the overall tunneling current as a function of the applied field. Our model explains the steplike features observed in high field tunneling experiments involving composite cathodes that incorporate electronic quantum-confinement regions. In order to assess the validity of our model and to show the apparition of the steplike features in field-emission experiments, we have used the usual diode configuration to obtain current-voltage characteristics from a composite cathode. The theoretical model presented in this paper shows qualitative agreement with the experimental data.

The synthesis of high-quality nanomaterials depends on the efficiency of the catalyst and the growth temperature. To produce high-quality material, high-growth temperatures (often up to 1000 °C) are regularly required and this can limit possible applications, especially where temperature sensitive substrates or tight thermal budgets are present. In this study, we show that high-quality catalyzed nanomaterial growth at low substrate temperatures is possible by efficient coupling of energy directly into the catalyst particles by an optical method. We demonstrate that using this photothermal-based chemical vapor deposition method that rapid growth (under 4 min, which includes catalyst pretreatment time) of high-density carbon nanotubes can be grown at substrate temperatures as low as 415 °C with proper catalyst heat treatment. The growth process results in nanotubes that are high quality, as judged by a range of structural, Raman, and electrical characterization techniques, and are compatible with the requirements for interconnect technology.

We demonstrate control of ZnO nanorod density for self-organized growth on ZnO buffer layers on Si by varying Zn supersaturation during the initial growth phase, thereby altering the competition between 2D and 1D growth modes. Higher initial supersaturation favours nanorods of diameter

The holy grail in terms of flat panel displays has been an inexpensive process for the production of large area 'hang on the wall' television that is based on an emissive technology. Electron field emission displays, in principle, should be able to give high quality pictures with good colour saturation, and, if suitable technologies for the production of cathodes over large areas were to be made available, at low cost. This requires a process technology where temperatures must be maintained below 450/spl deg/C throughout the entire production cycle to be consistent with the softening temperature of display glass. In this paper we propose three possible routes for nanoscale engineering of large area cathodes using low temperature processing that can be integrated into a display technology. The first process is based on carbon nanotube-polymer composites that can be screen printed over large areas and show electron field emission properties comparable with some of the best aligned nanotube arrays. The second process is based on the large area growth of carbon nanofibres directly onto substrates held at temperatures ranging from room temperature to 300/spl deg/C, thereby making it possible to use inexpensive substrates. The third process is based on the use of excimer laser processing of amorphous silicon for the production of lithography-free large area three terminal nanocrystalline silicon substrates. Each route has its own advantages and flexibility in terms of incorporation into an existing display technology. The harnessing of these synergies will be highlighted together with the properties of the cathodes developed for the differing technologies.

Electron spin resonance (ESR) and Raman spectra measurements are carried out on a-C:H and a-C:H:N films both as grown and implanted with W and Ni ions with doses ranged from 0.5×1015 to 1.2×1016 cm2. The as-grown films have small concentration of paramagnetic centers with a spin density Ns of 1017 cm3. Upon implantation a significant increase in Ns of (0.522)×1019 cm3 centers with g(Si) = 2.0055 and g(C) = 2.0025 was observed. These defects are ascribed to dangling bonds in the silicon substrate and in the carbon film, respectively. The correlation between variation of Ns value with implantation dose and behavior of D and G band position and their intensity ratio in the visible Raman spectra is observed. The effects are attributed to changes in the sp2sp3 systems and hydrogen loss due to ion induced annealing of the carbon films at high ion doses. The temperature and concentration dependencies of the ESR line shape and linewidth are explained using the mechanism of motional narrowing over the temperature range 4.2300 K. Low temperature anisotropy of the g value is found in the ESR spectra and is explained as arising from the dipoledipole interaction in the infinitely thin films.

The electronic properties of disordered carbon-based materials can be discussed in terms of the clustering of the sp2 carbon phase and delocalization of the electron wave function. In smooth amorphous carbon thin films this results in a mixed phase material of conductive sp2 clusters embedded in an electrically insulating sp3 matrix. The delocalization of the electron wave function associated with the sp2 clusters is shown to play an important role in understanding many of the electronic and optical properties of the films. It is demonstrated that the extent of the electron delocalization and clustering can be estimated using magnetic resonance methods. Evidence for delocalization in a range of carbon-based materials such as diamond-like carbon thin films produced by chemical vapour deposition, nanostructured carbon produced by pulsed laser ablation and ultrananocrystalline diamond is presented.

The validity of the cubic crystal field (CCF) approximation for the interpretation of the magnetic resonance properties of the Er3+ ion in crystal fields with tetragonal and trigonal symmetry is examined. The ground state paramagnetic resonance principal g values are explicitly calculated in terms of the cubic crystal field eigenstates as a function of axial crystal field strength. It is shown that, depending on the ground state crystal field eigenstate, the widely accepted CCF approximation of simply taking the average of the trace of the g tensor and equating it to the g value found in cubic symmetry can lead to a misinterpretation of the ground state Stark level and the lattice coordination of the ion. The implications for experimentally reported results are discussed.

We have measured the sub-THz electrical response of screen printed carbon nanotube-poly(methyl methacrylate) polymer composites up to 220 GHz. The measured electrical losses using mm long coplanar waveguide geometries averaged as low as 0.15 dB/mm in the frequency range 40 GHz–110 GHz and showed a reduction in signal loss with increasing frequency; a behaviour opposite to that found in conventional metallic conductors. Between 140 and 220 GHz, the electrical losses averaged 0.28 dB/mm. We show that the low electrical losses are associated with the capacitive coupling between the nanotubes and discuss potential high frequency applications.

Two-dimensional materials are one of the most active areas of nanomaterials research. Here we report the structural stability, electronic and vibrational properties of different monolayer configurations of the group IV elemental materials silicene and germanene. The structure of the stable configuration is calculated and for planar and low (

In situ three terminal electron field emission characterization of an isolated multiwalled carbon nanotube has been performed, where both anode and gate electrodes are attached to high precision piezodrivers. All measurements are performed in a scanning electron microscope allowing accurate knowledge of the local environment of the nanotube to be obtained. It is shown that the presence of the grounded gate electrode screens the applied field by approximately 32%. This technique in positioning the gate and anode electrodes allows for an estimate of the gate transparency factor and demonstrates characterization of individual carbon nanotubes without the need for fabrication of arrays of emitters.

This chapter focuses on the evolution of a carbon (C) thin films and maps out the significant contributions to the field by numerous research laboratories. The impact of the microstructure and growth on the optical and electrical properties is also examined in the chapter. New results show how ion implantation allows a methodology to delocalize gap states within these films. Carbon is unique in its structure by being able to form one of the strongest materials known to man—diamond—or one that is soft—graphite—by virtue of the way in which each atom bonds to another. All of these variations are made possible by the three different bond hybridizations that are available to carbon. Diamond-like carbon (DLC) should generally be reserved for polycrystalline or nanocrystalline carbon films, whereas amorphous carbon films should generally fall into the categories of polymer such as amorphous carbon (PAC), graphite-like amorphous carbon (GAC), diamond-like amorphous carbon (DAC), tetrahedral amorphous carbon (TAC), and nano-composite amorphous carbon (NAC).

In this paper, a novel terahertz (THz) spectroscopy technique and a new graphene-based sensor is proposed. The proposed sensor consists of a graphene-based metasurface (MS) that operates in reflection mode over a broad range of frequency band (0.2 -6 THz) and can detect relative permittivity of up to 4 with a resolution of 0.1 and a thickness ranging from 5 μm to 600 μm with a resolution of 0.5 μm. To the best of author’s knowledge, such a THz sensor with such capabilities has not been reported yet. Additionally, an equivalent circuit of the novel unit cell is derived and compared with two conventional grooved structures to showcase the superiority of the proposed unit cell. The proposed spectroscopy technique utilizes some unique spectral features of a broadband reflection wave including Accumulated Spectral power (ASP) and Averaged Group Delay (AGD), which are independent to resonance frequencies and can operate over a broad range of spectrum. ASP and AGD can be combined to analyse the magnitude and phase of the reflection diagram as a coherent technique for sensing purposes. This enables the capability to distinguish between different analytes with high precision which, to the best of author’s knowledge, has been accomplished for the first time.

Two-dimensional (2D) Ruddlesden–Popper perovskites (RPPs) of the form PEA2Pb1–x Sn x I4 can be used as the tunable active layer in photovoltaics, as the passivating layer for 3D perovskite photovoltaics or in light emitting diodes. Here, we show a nonlinear band gap behavior with Sn content in mixed phase 2D RPPs. Density functional theory calculations (with and without spin–orbit coupling) are employed to study the effects of the short-range ordering of Pb and Sn in PEA2Pb1–x Sn x I4 compositions with x = 0, 0.25, 0.5, 0.75, and 1. Analysis of the partial density of states shows that the energy mismatch of the Pb 6s and Sn 5s states in the valence band maximum determines the nonlinearity of the band gap, leading to a bowing parameter of 0.35–0.38 eV. This research provides a critical insight for the design of future metal alloy 2D perovskite materials. The positions of the tunable energy band discontinuity may point to intraband transitions of interest to device engineers.

Excited state design is an efficient approach towards new applications in molecular electronics spanning solar cells, artificial photosynthesis and biomedical diagnostics. Ruthenium (II)-polypiridyl based complexes are an example of molecular building blocks with tunable single and dual wavelength emission which can be controlled by excited state engineering via selective ligand modification. Here we investigate three new heteroleptic [Ru(bpy)2X]+ complex ions, where X represents pyridinyl or pyrazinyl derivatives of diazolates, providing tunable emission in the visible and infrared region. The dual emission is shown to arise from the presence of two excited states consisting of a triplet metal-to-ligand charge transfer state localized on a bipyridine ligand - 3MLCT (bpy), and either a state that is entirely localized on the X ligand or is partially delocalized also spanning part of the bipyridine ligands - 3MLCT(X). By a suitable choice of the X ligand, emission from 3MLCT(bpy) and 3MLCT (X) states can be rationally varied between 743 - 865 nm and from 555 - 679 nm, respectively. An increase in the nitrogen content of the six-membered ring of the X ligand results in a blueshift of the 3MLCT(bpy) emission but a redshift for the 3MLCT (X) emission. The wavelength difference between 3MLCT(bpy) and 3MLCT (X) emissions that can be tuned from 84–310 nm and is proportional to the difference in LUMOs energies (reduction potentials) of the isolated ligands. Our study provides key information towards new routes for the design of optically active dual wavelength molecular emitters.

An enhancement in the electrical performance of low temperature screen-printed silver nanoparticles (nAg) has been measured at frequencies up to 220 GHz. We show that for frequencies above 80 GHz the electrical losses in coplanar waveguide structures fabricated using nAg at 350 °C are lower than those found in conventional thick film Ag conductors consisting of micrometer-sized grains and fabricated at 850 °C. The improved electrical performance is attributed to the better packing of the silver nanoparticles resulting in lower surface roughness by a factor of 3. We discuss how the use of silver nanoparticles offers new routes to high frequency applications on temperature sensitive conformal substrates and in sub-THz metamaterials.

Nanoporous carbon films were deposited by 248 nm pulsed laser ablation of a graphite target in different background pressures of argon (P). The morphology changed from smooth, high-density amorphous carbon films at P = 20 mTorr to ultra-low density nanoporous material at P = 380 mTorr. Subsequently, the nanostructural, chemical and electrical properties of metal containing nanoporous carbon samples were investigated by ablating graphite targets containing known contents of Ni and Co. We demonstrate how the ablation plume dynamics affect both the nanostructure of the material and the incorporation of metal atoms. The suitability of these functionalised ultra-low density materials for gas sensing applications is discussed. © 2005 Materials Research Society.

The results of ab initio density functional theory calculations of molecular physisorption on a number of different adsorption sites on a graphene sheet and on a (10, 0) single walled carbon nanotube are discussed. Both the Vosko-Wilk-Nusair (VWN) local density approximation (LDA) functional and the Perdew-Wang (PW91) generalized gradient approximation (GGA) functional were employed in calculating the binding energy of a hydrogen molecule to the appropriate carbon nanostructure as well as the optimal molecule – nanostructure separation. Both exterior and interior nanotube adsorption sites were examined and it is shown that the binding energy associated with interior adsorption sites is larger than exterior adsorption on the nanotube or onto the graphene layer. The use of carbon nanostructures for hydrogen storage is also discussed.

In situ three terminal electron field emission characterization of an isolated multiwalled carbon nanotube has been performed, where both anode and gate electrodes are attached to high precision piezodrivers. All measurements are performed in a scanning electron microscope allowing accurate knowledge of the local environment of the nanotube to be obtained. It is shown that the presence of the grounded gate electrode screens the applied field by approximately 32%. This technique in positioning the gate and anode electrodes allows for an estimate of the gate transparency factor and demonstrates characterization of individual carbon nanotubes without the need for fabrication of arrays of emitters.

In this review the effects of clustering associated with the sp 2 and sp 3 phases of amorphous carbon thin films are examined. We highlight that many of the optical and electronic properties of these films can be explained by consideration of disorder in the sp 2 phase. Within the context of topological and structural disorder, we explain the variation of the visible Raman line width, Raman shift, Tauc gap and Urbach energy as a function of deposition conditions. We further go on to describe how intra-sp 2 cluster interactions are responsible for the narrowing of the electron paramagnetic resonance line width with increasing spin density and how this intracluster interaction can be extended to the intercluster transport properties, in particular, for electron field emission from the films. We also examine how the mechanical properties of carbon films are affected by clustering which can be enhanced by thermal annealing.

The effects of electrical current stressing on the field emission characteristics of hydrogenated amorphous carbon (a-C:H) thin films are reported. In these a-C:H films an initial conditioning treatment of the film is often required before the onset of stable emission and only after several voltage cycles do the values of the threshold field tend to converge. By stressing of the film by applying a predetermined current through the film, the initial conditioning treatment can be removed and stable and reproducible emission observed. Retesting of the current stressed films shows that the films remain fully conditioned provided a sufficiently high stress current was initially used.

Visible room-temperature photoluminescence (PL) was observed from hydrogen-free nanostructured amorphous carbon films deposited by pulsed laser ablation in different background pressures of argon (PAr). By varying PAr from 5 to 340 mTorr, the film morphology changed from smooth to rough and at the highest pressures, low-density filamentary growth was observed. Over the same pressure regime an increase in the ordering of s p2 bonded C content was observed using visible Raman spectroscopy. The origin of the PL is discussed in terms of improved carrier localization within an increased s p2 rich phase. © 2004 American Institute of Physics.

A comparison of the field emission properties of exposed nanotubes lying on a tipped carbon nanorope, with the emission properties from a sharpened iron tip of similar dimensions is performed. By varying the electrode separation it is observed that the threshold field for emission for both structures decreases as the electrode separation initially increases; however, for sufficiently large electrode separations, the threshold field is observed to reach an asymptotic value. Our results show that the field enhancement factor is fundamentally associated with the electrode separation, and depending on the experimental conditions in order to obtain a true value for electric field a set of alternative definitions for enhancement factors is required. We further confirm our experimental synopsis by simulation of the local electrostatic field which gives results similar to those obtained experimentally.

The optical emission from electronically excited C species in the ablation plume following the short (ns) and ultrashort (fs) UV pulsed laser ablation of graphite is studied. Wavelength, time and spatially resolved imaging of the plume, in background pressures of inert gases such as argon and helium, is performed. Analysis of images of optical emission from C+* ions and C-2(*) radicals, yielded estimates of the apparent velocity of emitting species, which appear to arise both from the initial ablation event and, in the presence of background gas, mainly from impact excitation. At elevated background pressures of argon (P-Ar), the formation and propagation of a shock wave is observed for ns pulses, whereas for fs pulses, the propagation of two shock waves is observed. During fs ablation, the first shock wave we associate with an initial burst of highly energetic/electronically excited ablated components, indicative of an enhanced fraction of non-thermal ejection mechanisms when compared with ns ablation. The second shock wave we associate with subsequently ejected, slower moving, material. Concurrent with the plume dynamics investigations, nanostructured amorphous carbon materials were deposited by collecting the ablated material. By varying P-Ar from 5 to 340 mTorr, the film morphology could be changed from mirror smooth, through a rough nanostructured phase and, at the highest background pressures for ns pulses, to a low density cluster-assembled material. The evident correlations between the film structure, the mean velocities of the emitting C species, and their respective dependences upon P-Ar are discussed for both pulse durations. In addition, we comment on the effect of observed initial plume dynamics on the subsequent C cluster formation in the expanding plume.

The catalytic growth of carbon nanotubes by excimer laser nanostructuring (LN) of nickel films was investigated. The thin films were first deposited by pulsed laser ablation (PLA). The plasma enhanced chemical vapor deposition (PECVD) was used to grow the multiwall carbon nanotubes. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used for the study of the films. It was observed that the combination of excimer laser growth and LN of Ni films was useful for the preparation of substrates with well-defined, discrete, nano-scale catalyst particles.

The growth and evolution of nanometre-sized Ni metal islands deposited under low-temperature non-ultra high vacuum conditions as a function of metal layer thickness, growth temperature and time is reported. The temperature of formation of the islands has been intentionally kept low for possible applications in flat panel display technology and also to act as a catalyst for carbon nanotube growth. It is shown that the size and distribution of the islands depends critically on the annealing temperature and the initial thickness of the metal layer. The mechanism of formation of the islands is described in terms of an Ostwald ripening mechanism of mass transport of either weakly bound individual Ni atoms or smaller clusters into larger more dispersed clusters.

Electron field emission measurements have been made on multiwall arc discharge carbon nanotubes embedded in a conjugated polymer host. Electron emission at low nanotube content is observed and attributed to an enhancement of the applied electric field at the polymer/nanotube/vacuum interface where the electron supply through the film is attributed to fluctuation induced tunneling in a disordered percolation network. A high network resistance is attributed to a polymer coating surrounding each nanotube, resulting in high resistance nanotube-polymer-nanotube tunnel junctions. The potential use of carbon nanotube-polymer composites for field emission based displays is also discussed.

The reactive ion etching of quartz and Pyrex substrates was carried out using CF /Ar and CF /O gas mixtures in a combined radio frequency (rf)/microwave (μw) plasma. It was observed that the etch rate and the surface morphology of the etched regions depended on the gas mixture (CF /Ar or CF /O ), the relative concentration of CF in the gas mixture, the rf power (and the associated self-induced bias) and microwave power. An etch rate of 95 nm/min for quartz was achieved. For samples covered with a thin metal layer, ex situ high resolution scanning electron microscopy and atomic force microscopy imaging indicated that, during etching, surface roughness is produced on the surface beneath the thin metallic mask. Near vertical sidewalls with a taper angle greater than 80° and smooth etched surfaces at the nanometric scale were fabricated by carefully controlling the etching parameters and the masking technique. A simulation of the electrostatic field distribution was carried out to understand the etching process using these masks for the fabrication of high definition features. © 2002 American Institute of Physics.

The origin of low threshold field-emission (threshold field 1.25 V/μm) in nanocrystalline diamond-like carbon (nc-DLC) thin films is examined. The introduction of nitrogen and thermal annealing are both observed to change the threshold field and these changes are correlated with changes to the film microstructure. A range of different techniques including micro-Raman and infrared spectroscopy, X-ray diffraction, electron microscopy, energy-dispersive X-ray analysis and time-of-flight-secondary ion mass spectroscopy are used to examine the properties of the films. A comparison of the field emission properties of nc-DLC films with atomically smooth amorphous DLC (a-DLC) films reveals that nc-DLC films have lower threshold fields. Our results show that nc-DLC can be a good candidate for large area field emission display panels and cold cathode emission devices. © 2012 American Chemical Society.

The nanostructure of amorphous carbon thin films is described in terms of a disordered nanometer-sized conductive sp2 phase embedded in an electrically insulating sp3 matrix. It is shown that the degree of clustering and disorder within the sp2 phase plays a determining role in the electronic properties of these films. Clustering of the sp2 phase is shown to be important in explaining several experimental results including the reduction of the electron spin resonance linewidth with increasing spin density and the dispersion associated with the width of the Raman active G band. The influence of structural disorder, associated with sp2 clusters of similar size, and topological disorder, due to undistorted clusters of different sizes, on both spin density and Raman measurements, is discussed. An extension of this description to intercluster interactions to explain some of the electrical transport and electron field emission behavior is also presented.

he mechanisms controlling the nanostructuring of thin metal-on-oxide films by nanosecond pulsed excimer lasers are investigated. When permitted by the interfacial energetics, the breakup of the metal film into nanoscale islands is observed. A range of metals (Au, Ag, Mo, Ni, Ti, and Zn) with differing physical and thermodynamic properties, and differing tendencies for oxide formation, are investigated. The nature of the interfacial metal-substrate interaction, the thermal conductivity of the substrate, and the initial thickness of the metal film are all shown to be of importance when discussing the mechanism for nanoscale island formation under high fluence irradiation. It is postulated that the resulting nanoparticle size distribution is influenced by the surface roughness of the initial film and the Rayleigh instability criterion. The results obtained are compared with simulations of the heat transfer through the film in order to further elucidate the mechanisms. The results are expected to be applicable to the laser induced melting of a large range of different materials, where poor wetting of substrate by the liquid phase is observed.

Pd/Co-based metal-filled carbon nanotubes (MF-CNTs) were synthesized by a microwave plasma-enhanced chemical vapor deposition method using a bias-enhanced growth technique. Pd/Co-based MF-CNTs were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) electron energy loss spectroscopy (EELS), and Raman spectroscopy. MF-CNTs were well-aligned and uniform in size on a Si substrate. Both multiwall nanotube carbon nanotubes (CNTs) and herringbone (or stacked cups structure) structures were observed. High-resolution TEM revealed that MF-CNTs were composed of highly ordered graphite layers, and the elemental maps of EELS indicate that both Co and Pd metals are present inside the nanotubes. TEM results clearly showed that both Pd and Co metals were successfully encapsulated into the CNTs. We observed a low value for the Raman intensity ratio between D (1355 cm(-1)) and G (1590 cm(-1)) bands with no shift of the G-peak position and no broadening of the G-peak, indicative of high-quality Pd/Co-based MF-CNTs. Based on TEM characterization, we propose a description for the encapsulating mechanisms.

The formation of Ni nanostructures to act as catalysts in the growth of carbon nanotubes is reported. The changes in the surface morphology of Ni produced by three methods - thermal evaporation and annealing of thin films, pulsed laser ablation and annealing of Ni, and the use of metal containing macromolecules - have been investigated by atomic force microscopy and scanning electron microscopy. In the case of thermal annealing of thin metal films in the temperature range 300-500oC we observe an increase in the mean diameter of the islands formed, accompanied by a reduction in the mean island density with increasing temperature. We attribute this effect to mass transport of weakly bound individual Ni atoms and/or small island clusters across the surface to form larger isolated islands, in a process similar to Ostwald ripening. Using a pulsed KrF excimer laser for ablation of a Ni target we show that nanometre smooth Ni thin films can be produced provided a sufficient number of laser shots is used. The surface morphology of these smooth films can then be altered by laser annealing to form Ni droplets. It is found that the mean diameter of the Ni droplets depends not only on the initial Ni thickness but also the laser fluence. It is also found that the nanostructuring of the film depends on the presence of an oxide under layer, with a higher fluence required on thinner oxides and no nanostructuring observed on bare Si. Finally, we show that Ni nanostructuring can be formed by suitable annealing of a Ni containing aqueous dendrimer solutions.

Electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy have been used to examine the structure and optical properties of erbium-impurity complexes formed in float-zone Si by multiple-energy implants at 77 K of Er together with either O or F. After implantation a 2-μm-thick amorphous layer was formed containing an almost uniform concentration of Er (1019/cm3)and O (3×1019/cm3 or 1020/cm3) or F (1020/cm3). Samples were annealed in nitrogen at 450 °C for 30 min (treatment A), treatment A+620 °C for 3 h (treatment B), treatment B+900 °C for 30 s (treatment C) or treatment B+900 °C for 30 min (treatment D). Samples coimplanted to have 3×1019O/cm3 and subject to treatment C show a broad line anisotropic EPR spectrum. These samples have the most intense low-temperature PL spectrum containing several sharp peaks attributed to Er3+ in sites with predominantly cubic Td symmetry. Increasing the O concentration to 1020/cm3 produces sharp line EPR spectra the strongest of which are attributed to two Er3+ centers having monoclinic C1h and trigonal symmetry. The principal g values and tilt angle for the monoclinic centers are g1=0.80, g2=5.45, g3=12.60, τ=57.3°, g∥=0.69, and g⊥=3.24 for the trigonal centers. The low-temperature PL spectrum from this sample showed additional sharp lines but the total intensity is reduced when compared to the sample with 3×1019O/cm3. For the sample containing 1020O/cm3 at least four distinct centers are observed by EPR after treatment B but after treatment D no EPR spectrum is observed. The PL spectra are also observed to change depending on the specific anneal treatment but even after treatment D, Er-related PL is still observed. Samples containing 1020F/cm3 and annealed with either treatment B or C produced an EPR spectrum attributed to Er3+in a site of monoclinic C1h symmetry with g1=1.36, g2=9.65, g3=7.91, and τ=79.1°.Tentative models for the structures of Er-impurity complexes are presented and the relationship between the EPR-active and PL-active centers is discussed.

The observation and origin of photoconductivity in high base pressure (∼10−3 Torr) grown nitrogen incorporated hydrogenated amorphous carbon (a-C:H:N) thin films is reported. The magnitude of conductivity at room temperature was measured to increase by nearly two orders of magnitude and exhibits a maximum ratio of photoconductivity to dark conductivity of 1.5 as the nitrogen content increased to 15.1 at. %. X-ray photoelectron spectroscopy, micro-Raman spectroscopy, and Fourier transform infrared spectroscopy reveal enhanced sp2 bonding at higher nitrogen contents. Residual film stress, Tauc band gap, hardness, and elastic modulus are all found to decrease with addition of nitrogen. The electrical characteristics suggest the creation of a-C:H:N/p-Si heterojunction diodes having rectifying behavior. The conductivity and electrical characteristics are discussed in term of band model, and the results show that high quality a-C:H:N films can be grown at high base pressures with properties comparable to those grown at low base pressures.

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.

We report on the evolution of the electronic structure with partial oxidation state of tin and iodine from ASnI¬3 to model a space charged region¬, where A = CH3NH3(MA), CH(NH2)2 (FA) or Cs, with a view to develop stable long-term non-toxic halide-perovskite solar cells. Although charge cannot be directly removed from specific elements, we show reduction of the charge primarily of SnI3 in a hypothetical [ASnI3]2+ unit cell is calculated due to the valence band edge being dominated by Sn 5s and I 5p anti-bonding states, accompanied by a reduction in unit cell volume in [ASnI3]2+ compared to ASnI3. Band structure calculations show semiconducting behaviour in ASnI3, with metallic behaviour in [ASnII3]2+; a similar behaviour is also found for APbI3 and [APbI3]2+, where the Pb atoms partial charge than Sn. This research al-lows for the analysis of localised charged regions, directing the ultimate long-term electronic stability in perovskite solar absorbers, such as interface recombination and deep trap states.

A comparison of the field emission properties of exposed nanotubes lying on a tipped carbon nanorope, with the emission properties from a sharpened iron tip of similar dimensions is performed. By varying the electrode separation it is observed that the threshold field for emission for both structures decreases as the electrode separation initially increases; however, for sufficiently large electrode separations, the threshold field is observed to reach an asymptotic value. Our results show that the field enhancement factor is fundamentally associated with the electrode separation, and depending on the experimental conditions in order to obtain a true value for electric field a set of alternative definitions for enhancement factors is required. We further confirm our experimental synopsis by simulation of the local electrostatic field which gives results similar to those obtained experimentally.

Visible room-temperature photoluminescence (PL) was observed from hydrogen-free nanostructured amorphous carbon films deposited by pulsed laser ablation in different background pressures of argon (PAr). By varying PAr from 5 to 340 mTorr, the film morphology changed from smooth to rough and at the highest pressures, low-density filamentary growth was observed. Over the same pressure regime an increase in the ordering of sp2 bonded C content was observed using visible Raman spectroscopy. The origin of the PL is discussed in terms of improved carrier localization within an increased sp2 rich phase.

We report the electrical responses of water vapour and O2 adsorption onto macroscopic multi-walled carbon nanotube (MWCNT) ropes, and compare the results with mats of acid-treated MWCNTs on SiO2 substrates in order to investigate the importance of oxygen-containing defects on CNTs. In the outgassed state both carbon nanotube (CNT) materials exhibit rapid changes in electrical resistance when exposed to dry air, humid air or water vapour at standard temperature and pressure (STP). The measured electrical responses are highly reversible at STP when cycled between humid air, vacuum and dry air. We report a decrease in resistance for the CNT materials in dry air, attributed to O2 p-type doping of the CNTs, whereas there is an increase in resistance when exposed to a humid environment. This latter effect is attributed to the formation of hydrogen bonding from the polar water molecules with the oxygen-containing defects on the CNTs. Our observations indicate that the increase in electrical resistance upon water absorption affects a reduction of the electron-withdrawing power of the oxygen-containing defect groups, thus leading to a reduced hole carrier concentration in the p-type nanotubes.

The reactive ion etching of quartz and Pyrex substrates was carried out using CF4/Ar and CF4/O2 gas mixtures in a combined radio frequency (rf)/microwave (µw) plasma. It was observed that the etch rate and the surface morphology of the etched regions depended on the gas mixture (CF4/Ar or CF4/O2), the relative concentration of CF4 in the gas mixture, the rf power (and the associated self-induced bias) and microwave power. An etch rate of 95 nm/min for quartz was achieved. For samples covered with a thin metal layer, ex situ high resolution scanning electron microscopy and atomic force microscopy imaging indicated that, during etching, surface roughness is produced on the surface beneath the thin metallic mask. Near vertical sidewalls with a taper angle greater than 80° and smooth etched surfaces at the nanometric scale were fabricated by carefully controlling the etching parameters and the masking technique. A simulation of the electrostatic field distribution was carried out to understand the etching process using these masks for the fabrication of high definition features.

Ultrasonication is the most widely used technique for the dispersion of a range of nanomaterials, but the intrinsic mechanism which leads to stable solutions is poorly understood with procedures quoted in the literature typically specifying only extrinsic parameters such as nominal electrical input power and exposure time. Here we present new insights into the dispersion mechanism of a representative nanomaterial, single-walled carbon nanotubes (SW-CNTs), using a novel up-scalable sonoreactor and an in situ technique for the measurement of acoustic cavitation activity during ultrasonication. We distinguish between stable cavitation, which leads to chemical modification of the surface of the CNTs, and inertial cavitation, which favors CNT exfoliation and length reduction. Efficient dispersion of CNTs in aqueous solution is found to be dominated by mechanical forces generated via inertial cavitation, which in turn depends critically on surfactant concentration. This study highlights that careful measurement and control of cavitation rather than blind application of input power is essential in the large volume production of nanomaterial dispersions with tailored properties.

Advances in lightweight, flexible and conformal electronic devices depend on materials that exhibit high electrical conductivity coupled with high mechanical strength. Defect-free graphene is one such material that satisfies both these requirements and which offers a range of attractive and tunable electrical, optoelectronic and plasmonic characteristics for devices that operate at microwave, THz, infra-red, or optical frequencies. Essential to the future success of such devices is therefore the ability to control the frequency dependent conductivity of graphene. Looking to accelerate the development of high frequency applications of graphene, here we demonstrate how readily accessible and processable organic and organometallic molecules can efficiently dope graphene to carrier densities in excess of 10^13 cm^-2 with conductivities at GHz frequencies in excess of 60 mS. In using the molecule F2-HCNQ, a high charge transfer (CT) of 0.5 electrons per adsorbed molecule is calculated resulting in p-type doping of graphene. N-type doping is achieved using cobaltocene and the sulphur containing molecule TTF with a CT of 0.41 and 0.24 electrons donated per adsorbed molecule, respectively. Efficient CT is associated with the interaction between the  electrons present in the molecule and in graphene. Calculation of the high frequency conductivity shows a dispersion-less behaviour of the real component of the conductivity over a wide range of GHz frequencies. Potential high frequency applications in graphene antennas and communications that can exploit these properties and the broader impacts of using molecular doping to modify functional materials which possess a low energy Dirac cone are also discussed.

The packing structure of bundled MoSI nanowires is investigated. Scanning and high-resolution transmission electron microscopy are used to determine both the nanowire structure and bundle superstructure. Shown is a high-resolution microscopy image of a small bundle. The image width is 8 nm. It is found that the nanowires pack in crystalline bundles defined by the P1 (#2) spacegroup.

From a semimetal to a wide gap semiconductor, from atomically flat diamond-like carbon thin films to nanostructured clustered films, from spherical fullerenes to high aspect ratio nanotubes, and from small molecules to long chain polymers, carbon is unique amongst the elements of the periodic table in taking such a wide variety of forms with such different structural and electronic properties. It is this richness and diversity that makes carbon as an electronic material so fascinating and was the impetus for the publication of this special issue of the Journal of Material Science: Materials in Electronics. The wide variety of material systems that are studied is equally matched by the different experimental and computational techniques and approaches that are used and this is reflected in the contributions from some of the internationally leading groups from around the world. At the outset the scene is set by the introduction of some of the figures of merit for a device oriented approach to the use of carbon as an electronic material. Both conjugated polymers and small molecules are discussed and the first paper concludes with some prospects for carbon nanotube based composites and the use of nanotubes in optical based devices such as photovoltaics devices and light emitting diodes. The second paper expands the discussion of the different bond hybridisations and in particular the role of disorder and how the clustering of the sp2 phase affects the electronic properties and electron emission from diamond-like carbon thin films. Field emission from carbon-nanotube conjugate polymer composites is also discussed and charge transportation is discussed in terms of percolation through the disordered polymer network. An in-depth experimental and theoretical discussion of electron transport in thin films is the subject of the third paper where localisation effects in the bandtails are shown to be important. From flat thin films to clustered carbon films, the fourth paper is a comprehensive discussion of nanostructured carbon produced from supersonic expansion. This paper presents a state of the art discussion of the synthesis, optical and electrical characteristics and possible future applications of clustered films. Staying with nanosized carbon, but moving to nanocrystalline diamond, the fifth paper discusses the growth and application to electron emission of sulphur containing nanocrystalline diamond. The paper discusses how the addition of sulphur results in modification of the sp2 phase and how this influences the electron emission. From ultrananocrystalline diamond to diamond itself, the sixth and seventh papers report on the use of computational methods to study diamond. The sixth paper describes some of the common impurities and impurity related defects that are of technological significance. An understanding of doping of diamond is crucial for diamond based electronics. Of equal importance is the role played by the surfaces of diamond in the origin of p-type conductivity. How fullerenes affect the doping on different terminated diamond surfaces is the subject of the seventh paper. Finally, an understanding of transport and the role that defects and degradation play in limiting potential devices are discussed in the final two papers of the special issue. Khan and colleagues describe the importance of charge balance and device lifetime in organic light emitting diodes as well as providing a viewpoint on the commercialisation of these materials for displays. The final paper deals with carrier transport in carbon nanotubes and how the intentionally incorporation of defects gives an unique opportunity to understand the factors that influence conduction. Clearly an intense international effort is on going in the many different fields for which this special issue is just a snapshot of high quality international leading research. It is therefore personal a pleasure for me to thank all the authors who have taken considerable time in putting together their papers. In addition, I would like to thank the Editor of the Journal of Materials Science: Materials in Electronics, Arthur Willoughby, for his help advice and to all those involve in the publishing of the special issue. I hope the reader will find the papers contained in the special issue to be of use and demonstrates the richness of carbon based electronic materials. Guest Editor David Carey Advanced Technology Institute University of Surrey, Guildford Email David.Carey@surrey.ac.uk

Some of the key challenges in the applications of graphene and carbon nanotubes are associated with their poor attachment to the substrate and poisoning of the catalyst by environmental contamination prior to the growth phase. Here we report a ‘protected catalyst’ technique which not only overcomes these challenges but also provides a new material production route compatible with many applications of carbon nanomaterials. The breakthrough technique involves capping the catalyst with a protective layer of a suitable material (examples include TiN, Cr, Ta) which protects the catalyst from environmental contaminants such as oxidation, etchant attack, etc., whilst maintaining carbon supply to the catalyst for the CVD growth of desired nanomaterial. A thin Fe catalyst layer remained protected due to the capping layer in the CF4 based reactive-ion-etching of SiO2. We show that the carbon nanostructures grown using this technique exhibit significantly improved adhesion to the substrate in sonication bath tests. We demonstrate the fabrication of 3D structures and CNT based vias in a buried catalyst arrangement using the protected catalyst technique. The technique also allows better control over various growth parameters such as number of graphene layers, growth rate, morphology, and structural quality.

Simulations of electric field enhancement factor, beta, for isolated metallic carbon nanotubes (CNT) are reported. We show that beta is dependent not only on the geometry of the tubes, but also on the location of the anode electrode. We also highlight the effect of field screening due to boundary conditions of the simulation package. Finally, we give an expression for beta as a function of CNT height, radius, and anode to cathode separation.

We report the electron field emission characteristics of functionalised single walled carbon nanotube – polymer composites produced by solution processing. We show that excellent electron emission can be obtained by using as little as 0.7% volume fraction of nanotubes in the composite. Furthermore by tailoring the nanotube concentration and type of polymer, improvements in the charge transfer through the composite can be obtained. The synthesis of well dispersed randomly oriented nanotube - polymer composites by solution processing allows the development of carbon nanotube based large area cathodes produced using a scaleable technology. The relative insensitivity of the cathode’s field emission characteristics to the electrical conductivity of the composite is also discussed.

The validity of the cubic crystal field (CCF) approximation for the interpretation of the magnetic resonance properties of the Er3+ ion in crystal fields with tetragonal and trigonal symmetry is examined. The ground state paramagnetic resonance principal g values are explicitly calculated in terms of the cubic crystal field eigenstates as a function of axial crystal field strength. It is shown that, depending on the ground state crystal field eigenstate, the widely accepted CCF approximation of simply taking the average of the trace of the g tensor and equating it to the g value found in cubic symmetry can lead to a misinterpretation of the ground state Stark level and the lattice coordination of the ion. The implications for experimentally reported results are discussed.