The pursuit of simultaneous ultra-high spatial and temporal resolution electron sources is a subject of intense research for a wide variety of applications in emerging light wave electronics and attosecond sciences. Recently, increasing research efforts on the production and integration of nanomaterials have projected wider scientific communities towards ultrafast electron emission devices that were hitherto manufacturable. Not only fascinating from the fundamental science point of view, such emerging electron emission systems offer an exciting platform for a wide variety of re-engineered as well as new applications. Here, the current state of the art in the field of ultrafast field electron emission, an in particular sub-optical-cycle field emission, using nanostructures is reviewed. Metallic nanotips, carbon nanotubes, and silicon nanotips, along with other promising nanomaterial platforms are considered alongside possible future research fields such materials may open up.
The carbon nanotube (CNT) cold cathode is an attractive choice for millimeter and terahertz vacuum electronic devices owning to its unique instant switch-on and high emission current density. A novel dual-gridded field emission architecture based on a CNT cold cathode is proposed here. CNTs are synthesized directly on the cathode surface. The first separating grid is attached to the CNT cathode surface to shape CNT cathode array. The second separating grid is responsible for controlled extraction of electrons from the CNT emitters. The cathode surface electric field distribution and beam transparency have been improved by …% compared to conventional planar devices. Furthermore, a high-compression-ratio dual-gridded CNT-based electron gun is designed to further increase the current density, and a 21 kV/50 mA electron beam is obtained with beam transparency of nearly 100%, along with a compression ratio of 39. A 0.22THz disk-loaded waveguide backward wave oscillator (BWO) based on this electron gun architecture has been realized theoretically with output power of 32 W. The results indicate that higher output power and higher frequency terahertz BWOs can be made using advanced nanomaterial-based cold cathodes.
Flexible electronics are being pursued as replacements for rigid consumer electronic products such as smartphones and tablets, as well as for wearable electronics, implantable medical devices, and RF-IDs. Such devices require flexible batteries with electrodes that maintain their electro-chemical performance during multiple bending cycles. These electrodes typically consist of an active battery material blend with a conductive additive and a binder. Whilst the choice of active battery material is typically dictated by the desired battery power and energy requirements, there is more freedom in changing the conductive additives to cope with strain induced during the bending of the flexible batteries. Here we compare the mechanical and electrical properties of free standing cathodes using lithium cobalt oxide (LiCoO2) as the active material and 10 to 20 wt% of amorphous carbon powder (CP) or carbon nanotubes (CNTs) as conductive additives. We found that the CNT based electrodes showed less crack formation during bending and have a Young’s modulus up to 30 times higher than CP electrodes (10 wt% loading). Further, the electrical resistance of pristine CNT electrodes is 10 times lower than CP electrodes (20 wt% loading). This difference further increases to a 28 times lower resistance for CNT films after 2000 bending cycles. These superior properties of CNT films were are reflected in the electrochemical tests, which show that after bending, only the electrodes with 20 wt% of CNTs remain operational. This study therefore highlights the importance of the conductive additives for developing reliable flexible batteries.
In this study we report on the electron transport in flexible-transparent polymer supported chemically doped chemical vapour deposited (CVD) graphene. We investigate the modified carrier transport following doping with various metal chlorides. An increase in the work function was noted for AuCl3-, FeCl3-, IrCl3-, and RhCl3-doping, whilst only SnCl2 doping showed a reduced work function. We attribute this to dopant-mediated charge transfer resulting in the formation of neutral atomic species. The neutral and near-neutral atomic populations produced metallic aggregates, with this agglomeration level critically dependent on the cohesive energy of the metallic component in each dopant. Micron-scale spatial conductivity mapping highlighted the spatially uniform low resistance of AuCl3-doped graphene. Local conductivity enhancements at grain boundaries and lattice defects within the as-synthesised polycrystalline graphene suggested that the dopant molecules tend to reside at lattice imperfections. Temperature dependent transport studies indicated that the shifted work function improved electrical conductivity due to the increase of barrier transparency between grain boundaries. Variable Range Hopping (VRH) dominated at temperatures <140 K in undoped graphene, whereas combined Nearest Neighbour Hopping (NNH) and diffusive transport appears to play a major role throughout the transport in all doped samples. The findings herein reveal that the underlying extended transport mechanisms associated with chemically doped CVD graphene transferred to polymer supports contrasts with the highly localised transport in undoped graphene.
Ultrafast electron pulses, combined with laser-pump and electron-probe technologies, allow ultrafast dynamics to be characterized in materials. However, the pursuit of simultaneous ultimate spatial and temporal resolution of microscopy and spectroscopy is largely subdued by the low monochromaticity of the electron pulses and their poor phase synchronization to the optical excitation pulses. Field-driven photoemission from metal tips provides high light-phase synchronization, but suffers large electron energy spreads (3–100 eV) as driven by a long wavelength laser (>800 nm). Here, ultrafast electron emission from carbon nanotubes (≈1 nm radius) excited by a 410 nm femtosecond laser is realized in the field-driven regime. In addition, the emitted electrons have great monochromaticity with energy spread as low as 0.25 eV. This great performance benefits from the extraordinarily high field enhancement and great stability of carbon nanotubes, superior to metal tips. The new nanotube-based ultrafast electron source opens exciting prospects for extending current characterization to sub-femtosecond temporal resolution as well as sub-nanometer spatial resolution.
A concurrently high beam current and high current density carbon nanotube (CNT) cold cathode electron gun is herein developed. A radial electron source has been realized, formed from CNTs synthesized directly on the side walls of a stainless steel truncated-cone electron gun. Experimental results evidenced a 35 kV/50 mA electron beam can achieve a beam transparency of nearly 100% through the use of double anodes and crossed electric and magnetic fields. A maximum beam current density of 3.5 A/cm2 was achieved. These results demonstrate the potential impact of coupling novel cold cathode gun architectures and emerging nanomaterials and their collective role in augmenting the performance of incumbent electron gun technologies, alongside allowing for the realization new types of field emission vacuum electron radiation sources.
Here we investigate, through parametrically optimized macroscale simulations, the field electron emission from arrays of carbon nanotube (CNT)-coated Spindts towards the development of an emerging class of novel vacuum electron devices. The present study builds on empirical data gleaned from our recent experimental findings on the room temperature electron emission from large area CNT electron sources. We determine the field emission current of the present microstructures directly using particle in cell (PIC) software and present a new CNT cold cathode array variant which has been geometrically optimized to provide maximal emission current density, with current densities of up to 11.5 A/cm2 at low operational electric fields of 5.0 V/μm.
In this work we report on the fabrication of inductively coupled plasma (ICP) etched, diode-type, bulk molybdenum field emitter arrays. Emitter etching conditions as a function of etching mask geometry and process conditions were systematically investigated. For optimized uniformity, aspect ratios of > 10 were achieved, with 25.5 nm-radius tips realised for masks consisting of aperture arrays some 4.45 μm in diameter and whose field electron emission performance has been herein assessed.
With the rapidly increasing demands for ultrasensitive biodetection, the design and applications of new nano-scale materials for development of sensors based on optical and electrochemical transducers have attracted substantial interest. In particular, given the comparable sizes of nanomaterials and biomolecules, there exist plenty of opportunities to develop functional nanoprobes with biomolecules for highly sensitive and selective biosensing, shedding new light on cellular behaviour. Towards this aim, herein we interface cells with patterned nano-arrays of carbon nanofibers forming a nanosensor-cell construct. We show that such a construct is capable of electrochemically communicating with the intracellular environment.
Suppression of the hysteretic electron emission in one-dimensional nanomaterial-based electron sources remains a Critical barrier preventing their wide scale adoption in various vacuum electronics applications. Here we report on the suppressed hysteretic performance, and its photo-dependence from conformal poly-vinylpyrrolidone (PVP) encapsulated percolative Ag nanowire (NW)-based electron sources.
The field emission (FE) properties of carbon nanotube (CNT)-based cathodes have been investigated on nanostructured surfaces grown by plasma enhanced chemical vapor deposition. The FE angular properties and temporal stability of the emergent electron beam have been determined using a dedicated apparatus for cathodes of various architectures and geometries, characterized by scanning electron microscopy and I-V measurements. The angular electron beam divergence and time instability at the extraction stage, which are crucial parameters in order to obtain high brilliance of FE-based-cathode electron sources, have been measured for electrons emitted by several regular architectures of vertically aligned arrays and critically compared to conventional disordered cathodes. The measured divergences strongly depend on the grid mesh. For regular arrays of individual CNT divergences from 2° to 5° have been obtained; in this specific case, measurements together with ray-tracing simulations suggest that the maximum emission angle is of the order of ±30° about the tube main axis. Larger divergences have been measured for electron beams emitted from honeycomb-structured cathodes (6°) and significantly broader angle distributions (10°) from disordered CNT surfaces. Emission current instabilities of the order of 1% for temporal stability studies conducted across a medium time scale (hours) have been noted for all cathodes consisting of a high number (104 and larger) of aligned CNTs, with the degree of stability being largely independent of the architecture.
The production of horizontally aligned carbon nanotubes offers a rapid means of realizing a myriad of self-assembled near-atom-scale technologies - from novel photonic crystals, to nanoscale transistors. The ability to reproducibly align anisotropic nanostructures has huge technological value. Here we review the present state-of-the-art in horizontal carbon nanotube alignment. For both in- and ex-situ approaches, we quantitatively assess the reported linear packing densities alongside the degree of alignment possible for each of these core methodologies.
This study reports on a facile and widely applicable method of transferring chemical vapour deposited (CVD) graphene uniformly onto optically transparent and mechanically flexible substrates using commercially available, low cost ultra-violet adhesive (UVA) and hot-press lamination (HPL). We report the adhesion potential between the graphene and the substrate and we compare these findings with those of the more commonly used coast polymer handler transfer processes. Graphene transferred with the two proposed methods showed lower surface energy and displayed a higher degree of adhesion (UVA: 4.40±1.09 N/m, HPL: 0.60±0.26 N/m) compared to equivalent CVD-graphene transferred using conventional poly-methyl methacrylate (PMMA: 0.44±0.06 N/m). The mechanical robustness of the transferred graphene was investigated by measuring the differential resistance as a function of bend angle and repeated bend-relax cycles across a range of bend radii. At a bend angle 100º and 2.5 mm bend radius, for both transfer techniques, the normalised resistance of graphene transferred on polyethylene terephthalate (PET) was around 80 times less than indium-tin-oxide on PET. After 104 bend cycles, the resistance of the transferred graphene on PET using UVA and HPL was found to be, on average, around 25.5 % and 8.1 % higher than PMMA-transferred graphene indicating that UVA- and HPL-transferred graphene are more strongly adhered compared to PMMA-transferred graphene. The robustness, in terms of maintained electrical performance upon mechanical fatigue, of the transferred graphene was around 60 times improved over ITO/PET upon many thousands of repeated bending stress cycles. Based on present production methods, the development of the next-generation of highly conformal, diverse form factor electronics, exploiting the emerging family of two-dimensional materials, necessitates the development of simple, low-cost, and mechanically robust transfer processes; the developed UVA and HPL approaches show significant potential and allow for large area compatible, near-room temperature transfer of graphene onto a diverse range of polymeric supports.
Carbon nanotubes, and the wider class of graphitic nanocarbons, have a proven potential as efficient field electron emission sources though the enhancement of their emission characteristics upon adlayer inclusion is little understood to date. Here we demonstrate a simple adlayer scheme capable of enhancing the native field emission properties of as-grown chemical vapour deposited carbon nanotubes through simultaneous geometric and electronic enhancement using a new class of nanotube-vacuum interface adlayer; anionic zwitterionic conjugated polyelectrolytes (ZCPEs). Adlayers preserve the beneficial geometries and high aspect ratios of these nanoscale one-dimensional structures whilst concurrently enhancing the emission performance. Though a wide set of adlayer materials have been considered elsewhere, of which almost all have exclusively focused on low work function metal over-coatings, here we report on the use of engineered polar conjugated polyelectrolyte adlayers. We find that our ZCPE adlayer mediates a fourfold decrease in free carrier concentration at the upper most emission surface which reduced the deleterious effects of nearest neighbour shielding common to highly-packed carbon nanotube based emitters. The dominant space charge limiting mechanism, and subsequent observed barrier transmission, was found to be consistent with Lampert-Rose, with potential specific deviation attributed to depopulation of local, optically active, trap states. The surface dipole, the magnitude of which relates intimately to the choice of side chain, effectively reduces the emitter-vacuum barrier increasing the transmission efficiency at lower electric fields. Whereas cationic ZCPEs, such as those with nitrogen side groups, will likely increase the turn-on field our evidence suggests that anionic polymers favour reduced turn-on fields, with our results highlighting a clear strategy for the use of engineered polar molecules, coupled to the wider family of novel nanocarbon allotropes, as a means to realise inexpensive and readily adopted high current density electron sources for use in high resolution microscopy and advanced metrology, next generation free electron lasers, desktop scale particle accelerators, along with high bandwidth travelling wave tubes and pulsed X-ray sources.
Multi-beam modulation in a carbon nanotube (CNT) cold cathode electron gun is herein investigated in order to develop miniaturized and fully-integrated vacuum electron devices. By exposing the electron source to a millimeter-wave signal the steady state field emission current density is efficiently modulated by the incident high-frequency (HF) electric field. Our simulation results of this multi-beam electron gun show that the field emission current density can be efficiently modulated by different incident frequency millimeter-waves. We find that the modulation depth is increased by enhancing the HF input power and anode operation voltage. The modulation frequency and phase of each electron beam can be controlled using a single millimeter-wave source and by simply adjusting the lateral distance between adjacent CNT cold cathodes.
This paper reports on a simple approach for fabricating high aspect ratio field emission arrays (FEAs) directly from bulk molybdenum substrates via the use of fluorine inductive-coupled-plasma (ICP) etching. Compared to the traditional Spindt array, through our outlined fabrication process all thin film interfaces have been eliminated reducing overall tip delamination during operation. The as-fabricated devices exhibited low turn-on electric fields (for I = 100 nA) of 1.21 V/μm. Arrays of more than 106 tips, with controlled inter-tip-pitches of 10 μm, have produced maximum currents of up to 140 μA (at 5.48 V/μm). Spatially uniform emission, that can be white light optically modulated, has been observed from the developed bulk Mo-FEAs making them promising for use in long-term, high beam current continuous emission applications.
Graphene is a promising material for transparent flexible electronics. In this study, we report on the development of a doping scheme for large-area chemical vapour deposited graphene transferred to large-area, optically transparent and mechanically flexible polymer supports. A transfer technique using ultra-violet adhesive (UVA) is outlined. The temporal stability of the sheet resistance and optical transparency following chemical doping with various metal chlorides (MexCly); gold chloride (AuCl3), ferric chloride (FeCl3), tin chloride (SnCl2), iridium chloride (IrCl3) and rhodium chloride (RhCl3), is subsequently detailed. The sheet resistance (RS) and 550 nm optical transparency (%T550) of the transferred undoped graphene were 3.5 kΩ/sq. (±0.2 kΩ/sq.) and 84.1% (±2.9%), respectively. Doping with AuCl3 showed a notable reduction in RS by some 71.4 % (to 0.93 kΩ/sq.) with a corresponding %T550 of 77.0%. After 200 hours’, exposure to STP air, the increase in RS was found to be negligible (△RSAuCl3 = 0.06 kΩ/sq.) indicating that, of the consider MexCly dopants, AuCl3 doping offered the highest degree of time stability under ambient conditions. There appears a tendency for RS to increase with time for the remaining metal chlorides studied. We attribute the observed temporal shift to dynamic desorption of the molecular dopants. We find that the desorption was most significant in FeCl3-doped samples (ΔГ/Г = -0.942), whilst, in contrast, after 200 hours in ambient, AuCl3-doped graphene showed only marginal desorption (ΔГ/Г = 0.075). We find that chemical doping of UVA transferred graphene is a particularly promising means for enhancing large-area graphene-based electrodes in order to offer a viable platform for next generation flexible optoelectronics.
Following the recent global excitement and investment in the emerging, and rapidly growing, classes of one and two-dimensional nanomaterials, we here present a perspective on one of the viable applications of such materials: field electron emission based x-ray sources. These devices, which have a notable history in medicine, security, industry and research, to date have almost exclusively incorporated thermionic electron sources. Since the middle of the last century, field emission based cathodes were demonstrated, but it is only recently that they have become practicable. We outline some of the technological achievements of the past two decades, and describe a number of the seminal contributions. We explore the foremost market hurdles hindering their roll-out and broader industrial adoption and summarise the recent progress in miniaturised, pulsed and multi-source devices.
The field electron emission performance of bulk, 1D, and 2D nanomaterials is here empirically compared in the largest metal-analysis of its type. No clear trends are noted between the turn-on electric field and maximum current density as a function of emitter work function, while a more pronounced correlation with the emitters dimensionality is noted. The turn-on field is found to be twice as large for bulk materials compared to 1D and 2D materials, empirically confirming the wider communities view that high aspect ratios, and highly perturbed surface morphologies allow for enhanced field electron emitters.
We report on the improved field emission performance of graphene foam (GF) following transient exposure to hydrogen plasma. The enhanced field emission mechanism associated with hydrogenation has been investigated using Fourier transform infrared spectroscopy, plasma spectrophotometry, Raman spectroscopy, and scanning electron microscopy. The observed enhanced electron emissionhas been attributed to an increase in the areal density of lattice defects and the formation of a partially hydrogenated, graphane-like material. The treated GF emitter demonstrated a much reduced macroscopic turn-on field (2.5 V μm-1), with an increased maximum current density from 0.21 mA cm-2 (pristine) to 8.27 mA cm-2 (treated). The treated GFs vertically orientated protrusions, after plasma etching, effectively increased the local electric field resulting in a 2.2-fold reduction in the turn-on electric field. The observed enhancement is further attributed to hydrogenation and the subsequent formation of a partially hydrogenated structured 2D material, which advantageously shifts the emitter work function. Alongside augmentation of the nominal crystallite size of the graphitic superstructure, surface bound species are believed to play a key role in the enhanced emission. The hydrogen plasma treatment was also noted to increase the emission spatial uniformity, with an approximate four times reduction in the per unit area variation in emission current density. Our findings suggest that plasma treatments, and particularly hydrogen and hydrogen-containing precursors, may provide an efficient, simple, and low cost means of realizing enhanced nanocarbon-based field emission devices via the engineered degradation of the nascent lattice, and adjustment of the surface work function.
In this letter, we present a fully complementary-metal-oxide-semiconductor (CMOS) compatible microelectromechanical system thermopile infrared (IR) detector employing vertically aligned multi-walled carbon nanotubes (CNT) as an advanced nano-engineered radiation absorbing material. The detector was fabricated using a commercial silicon-on-insulator (SOI) process with tungsten metallization, comprising a silicon thermopile and a tungsten resistive micro-heater, both embedded within a dielectric membrane formed by a deep-reactive ion etch following CMOS processing. In-situ CNT growth on the device was achieved by direct thermal chemical vapour deposition using the integrated micro-heater as a micro-reactor. The growth of the CNT absorption layer was verified through scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The functional effects of the nanostructured ad-layer were assessed by comparing CNT-coated thermopiles to uncoated thermopiles. Fourier transform IR spectroscopy showed that the radiation absorbing properties of the CNT adlayer significantly enhanced the absorptivity, compared with the uncoated thermopile, across the IR spectrum (3.0 μm–15.5 μm). This led to a four-fold amplification of the detected infrared signal (4.26 μm) in a CO2 non-dispersive-IR gas sensor system. The presence of the CNT layer was shown not to degrade the robustness of the uncoated devices, whilst the 50% modulation depth of the detector was only marginally reduced by 1.5 Hz. Moreover, we find that the 50% normalized absorption angular profile is subsequently more collimated by 8. Our results demonstrate the viability of a CNT-based SOI CMOS IR sensor for low cost air quality monitoring.
Since the discovery of X-rays over a century ago the techniques applied to the engineering of X-ray sources have remained relatively unchanged. From the inception of thermionic electron sources, which, due to simplicity of fabrication, remain central to almost all X-ray applications, there have been few fundamental technological advances. However, with the emergence of ever more demanding medical and inspection techniques, including computed tomography and tomosynthesis, security inspection, high throughput manufacturing and radiotherapy, has resulted in a considerable level of interest in the development of new fabrication methods. The use of conventional thermionic sources is limited by their slow temporal response and large physical size. In response, field electron emission has emerged as a promising alternative means of deriving a highly controllable electron beam of a well-defined distribution. When coupled to the burgeoning field of nanomaterials, and in particular, carbon nanotubes, such systems present a unique technological opportunity. This review provides a summary of the current state-of-the-art in carbon nanotube-based field emission X-ray sources. We detail the various fabrication techniques and functional advantages associated with their use, including the ability to produce ever smaller electron beam assembles, shaped cathodes, enhanced temporal stability and emergent fast-switching pulsed sources. We conclude with an overview of some of the commercial progress made towards the realisation of an innovative and disruptive technology.
Exploiting the anisotropic properties of aligned one-dimensional carbon nano-allotropes has proven useful in myriad applications, including: cold cathode fi eld emission, nanostructured ropes, nanoelectromechanical switched capacitive memory, composites and birefringent media. For many applications, it is important, however; that the native opto-electronic properties of the aligned nanostructured media remain unmodifi ed by the device fabrication processes. By investigating the chemical vapour deposition conditions of multi-walled carbon nanotubes we have developed a route to produce highly aligned free-standing carbon nanotube films by engineering a metastable, membrane-like morphology. Using an augmented Leonard-Jones formalism we elucidate the conditions required for membrane formation which we show depends critically on the nanotube length and packing density, dictated by the detailed growth conditions, in addition to entropic contributions associated with the detailed membrane morphology. We demonstrate an ultra-broadband polarising behavior of our binder- and substrate-less membranes, investigated in the wavelength range from 400 nm to 2.5 mm. Extinction ratios of up to 6.4 dB are measured, consistent with effective medium and full numerical simulations using index and absorption constants extracted from the Drude-Lorentz model. The free-standing metastable membranes represent a new paradigm in the design and fabrication of optical components in emerging supercontinuum light sources based on subwavelength periodic nanomaterials.
Here we experimentally study the microwave absorption and near-field radiation behavior of monolayer and few-layer, large-area CVD graphene in the C and X bands. Artificial stacking of CVD graphene reduces the sheet resistance, as verified by non-contact microwave cavity measurements and four-probe DC resistivity. The multilayer stacked graphene exhibits increased absorption determined by the total sheet resistance. The underlying mechanism could enable us to apply nanoscale graphene sheets as optically transparent radar absorbers. Near-field radiation measurements show that our present few-layer graphene patches with sheet resistance more than 600 Ω/sq exhibit no distinctive microwave resonance and radiate less electromagnetic power with increasing layers; however, our theoretical prediction suggests that for samples to be practical as microwave antennas, doped multilayer graphene with sheet resistance less than 10 Ω/sq is required.
The ability to accurately design carbon nanofibre (CN) field emitters with predictable electron emission characteristics will enable their use as electron sources in various applications such as microwave amplifiers, electron microscopy, parallel beam electron lithography and advanced Xray sources. Here, highly uniform CN arrays of controlled diameter, pitch and length were fabricated using plasma enhanced chemical vapour deposition and their individual emission characteristics and field enhancement factors were probed using scanning anode field emission mapping. For a pitch of 10 mm and a CN length of 5 mm, the directly measured enhancement factors of individual CNs was 242, which was in excellent agreement with conventional geometry estimates (240). We show here direct empirical evidence that in regular arrays of vertically aligned CNs the overall enhancement factor is reduced when the pitch between emitters is less than half the emitter height, in accordance to our electrostatic simulations. Individual emitters showed narrow Gaussian-like field enhancement distributions, in excellent agreement with electric field simulations.
The primary barrier to wider commercial adoption of graphene lies in reducing the sheet resistance of the transferred material without compromising its high broad-band optical transparency, ideally through the use of novel transfer techniques and doping strategies. Here, chemical vapour deposited graphene was uniformly transferred to polymer supports by thermal and ultraviolet (UV) approaches and the time-dependent evolution of the opto-electronic performance was assessed following exposure to three kinds of common dopants. Doping with FeCl3 and SnCl2 showed minor, and notably time unstable, enhancement in the σopt/σdc figure of merit, while AuCl3-doping markedly reduced the sheet resistance by 91.5% to 0.29 kΩ/ sq for thermally transferred samples and by 34.4% to 0.62 kΩ/sq for UV-transferred samples, offering a means of realising viable transparent flexible conductors that near the indium tin oxide benchmark.
A model of the graphene growth mechanism of chemical vapor deposition on platinum is proposed and verified by experiments. Surface catalysis and carbon segregation occur, respectively, at high and low temperatures in the process, representing the so-called balance and segregation regimes. Catalysis leads to self-limiting formation of large area monolayer graphene, whereas segregation results in multilayers, which evidently “grow from below.” By controlling kinetic factors, dominantly monolayer graphene whose high quality has been confirmed by quantum Hall measurement can be deposited on platinum with hydrogen-rich environment, quench cooling, tiny but continuous methane flow and about 1000°C growth temperature.
The development of transparent radio-frequency electronics has been limited, until recently, by the lack of suitable materials. Naturally thin and transparent graphene may lead to disruptive innovations in such applications. Here, we realize optically transparent broadband absorbers operating in the millimetre wave regime achieved by stacking graphene bearing quartz substrates on a ground plate. Broadband absorption is a result of mutually coupled Fabry-Perot resonators represented by each graphene-quartz substrate. An analytical model has been developed to predict the absorption performance and the angular dependence of the absorber. Using a repeated transfer-and-etch process, multilayer graphene was processed to control its surface resistivity. Millimetre wave reflectometer measurements of the stacked graphene-quartz absorbers demonstrated excellent broadband absorption of 90% with a 28% fractional bandwidth from 125–165 GHz. Our data suggests that the absorbers’ operation can also be extended to microwave and low-terahertz bands with negligible loss in performance.
The enhanced emission performance of a graphene/Mo hybrid gate electrode integrated into a nanocarbon fi eld emission micro-triode electron source is presented. Highly electron transparent gate electrodes are fabricated from chemical vapor deposited bilayer graphene transferred to Mo grids with experimental and simulated data, showing that liberated electrons efficiently traverse multi-layer graphene membranes with transparencies in excess of 50–68%. The graphene hybrid gates are shown to reduce the gate driving voltage by 1.1 kV, whilst increasing the electron transmission efficiency of the gate electrode significantly. Integrated intensity maps show that the electron beam angular dispersion is dramatically improved (87.9°) coupled with a 63% reduction in beam diameter. Impressive temporal stability is noted (<1.0%) with surprising negligible long-term damage to the graphene. A 34% increase in triode perveance and an amplifi cation factor 7.6 times that of conventional refractory metal grid gate electrode-based triodes are noted, thus demonstrating the excellent stability and suitability of graphene gates in micro-triode electron sources.
In the design of capacitive touch-screen panels, electrodes are patterned to improve touch sensitivity. In this paper, we analyze the relationship between electrode patterns and touch sensitivity. An approach is presented where simulations are used to measure the sensitivity of touch-screen panels based on capacitance changes for various electrode patterns. Touch sensitivity increases when the touch object is positioned in close proximity to fringing electric fields generated by the patterned electrodes. Three new electrode patterns are proposed to maximize field fringing in order to increase touch sensitivity by purely electrode patterning means. Simulations showed an increased touch sensitivity of up to 5.4%, as compared with the more conventional interlocking diamonds pattern. Here, we also report empirical findings for fabricated touch-screen panels.
This paper presents a novel method of using experimentally observed optical phenomena to reverseengineer a model of the carbon nanofiber-addressed liquid crystal microlens array (C-MLA) using Zemax. It presents the first images of the optical profile for the C-MLA along the optic axis. The first working optical models of the C-MLA have been developed by matching the simulation results to the experimental results. This approach bypasses the need to know the exact carbon nanofiber–liquid crystal interaction and can be easily adapted to other systems where the nature of an optical device is unknown.
Results show that the C-MLA behaves like a simple lensing system at 0.060–0.276 V/mm. In this lensing mode the C-MLA is successfully modeled as a reflective convex lens array intersecting with a flat reflective plane. The C-MLA at these field strengths exhibits characteristics of mostly spherical or low order aspheric arrays, with some aspects of high power aspherics. It also exhibits properties associated with varying lens apertures and strengths, which concur with previously theorized models based on E-field patterns. This work uniquely provides evidence demonstrating an apparent “rippling” of the liquid crystal texture at low field strengths, which were successfully reproduced using rippled Gaussianlike lens profiles.
Carbon nanostructures have been much sought after for cold-cathode field emission applications. Herein a printing technique is reported to controllably nanostructure chemical vapor deposited graphene into vertically standing fins. The method allows for the creation of regular arrays of bilayer graphene fins, with sharp ridges that, when printed onto gold electrodes, afford a new type of field emission electron source geometry. The approach affords tunable morphologies and excellent long term and cyclic stabilities.
Herein we report on the transport characteristics of rapid pulsed vacuum-arc thermally annealed, individual and network multi-walled carbon nanotubes. Substantially reduced defect densities (by at least an order of magnitude), measured by micro-Raman spectroscopy, and were achieved by partial reconstruction of the bamboo-type defects during thermal pulsing compared with more traditional single-pulse thermal annealing. Rapid pulsed annealed processed networks and individual multi-walled nanotubes showed a consistent increase in conductivity (of over a factor of five at room temperature), attributed to the reduced number density of resistive axial interfaces and, in the case of network samples, the possible formation of structural bonds between crossed nanotubes. Compared to the highly defective as-grown nanotubes, the pulsed annealed samples exhibited reduced temperature sensitivity in their transport characteristics signifying the dominance of scattering events from structural defects. Transport measurements in the annealed multi-walled nanotubes deviated from linear Ohmic, typically metallic, behavior to an increasingly semiconducting-like behavior attributed to thermally induced axial strains. Rapid pulsed annealed networks had an estimated band gap of 11.26 meV (as-grown; 6.17 meV), and this observed band gap enhancement was inherently more pronounced for individual nanotubes compared with the networks most likely attributed to mechanical pinning effect of the probing electrodes which possibly amplifies the strain induced band gap. In all instances the estimated room temperature band gaps increased by a factor of two. The gating performance of back-gated thin-film transistor structures verified that the observed weak semiconductivity (p-type) inferred from the transport characteristic at room temperature.
A solution processed aluminum-doped zinc oxide (AZO)/multi-walled carbon nanotube (MWCNT) nanocomposite thin film has been developed offering simultaneously high optical transparency and low electrical resistivity, with a conductivity figure of merit (σDC/σopt) of 75– better than PEDOT:PSS and many graphene derivatives. The reduction in sheet resistance of thin films of pristine MWCNTs is attributed to an increase in the conduction pathways within the sol–gel derived AZO matrix and reduced inter-MWCNT contact resistance. Films have been extensively characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffractometry (XRD), photoluminescence (PL), and ultraviolet–visible (UV–vis) spectroscopy.
Near-field measurements were performed at X-band frequencies for graphene on copper microstrip transmission lines. An improvement in radiation of 0.88 dB at 10.2 GHz is exhibited from the monolayer graphene antenna which has dc sheet resistivity of 985 Ω/sq. Emission characteristics were validated via ab initio simulations and compared to empirical findings of geometrically comparable copper patches. This study contributes to the current knowledge of the electronic properties of graphene.
In this paper, we demonstrate a micro-inkjet printing technique as a reproducible post-process for the deposition of carbon nanoparticles and fullerene adlayers onto fully CMOS compatible micro-electro-mechanical silicon-on-insulator infrared (IR) light sources to enhance their infrared emission. We show experimentally a significant increase in the infrared emission efficiency of the coated emitters. We numerically validate these findings with models suggesting a dominant performance increase for wavelengths <5.5 μm. Here, the bimodal size distribution in the diameter of the carbon nanoparticles, relative to the fullerenes, is an effective mediator towards topologically enhanced emittance of our miniaturised emitters. A 90% improvement in IR emission power density has been shown which we have rationalised with an increase in the mean thickness of the deposited carbon nanoparticle adlayer.
A plasma-enhanced chemical vapour deposition reactor has been developed to synthesis horizontally aligned carbon nanotubes. The width of the aligning sheath was modelled based on a collisionless, quasi-neutral, Child’s law ion sheath where these estimates were empirically validated by direct Langmuir probe measurements, thereby confirming the proposed reactors ability to extend the existing sheath fields by up to 7 mm. A 7 mbar growth atmosphere combined with a 25 W plasma permitted the concurrent growth and alignment of carbon nanotubes with electric fields of the order of 0.04 V μm−1 with linear packing densities of up to ~5×104cm−1. These results open up the potential for multi-directional in situ alignment of carbon nanotubes providing one viable route to the fabrication of many novel optoelectronic devices.
It has been claimed that graphene growth on copper by chemical vapor deposition is dominated by crystallization from the surface initially supersaturated with carbon adatoms, which implies that the growth is independent of hydrocarbon addition after the nucleation phase. Here, we present an alternative growth model based on our observations that oppose this claim. Our Gompertzian sigmoidal growth kinetics and secondary nucleation behavior support the postulate that the growth can be controlled by adsorption−desorption dynamics and the dispersive kinetic processes of catalytic dissociation and dehydrogenation of carbon precursors on copper.
The fabrication and functionality of a 21 cm graphene-based transverse electron emission display panel is presented. A screen-printed triode edge electron emission geometry has been developed based on chemical vapor deposited (CVD) graphene supported on vertically aligned carbon nanotubes (CNT) necessary to minimize electrostatic shielding induced by the proximal bulk substrate. Integrated ZnO tetrapod electron scatterers have been shown to increase the emission efficiency by more than 90%. Simulated electron trajectories validate the observed emission characteristics with driving voltages less than 60 V. Fabricated display panels have shown real-time video capabilities that are hysteresis free (<0.2%), have extremely stable lifetimes (<3% variation over 10 h continuous operation) in addition to rapid temporal responses (<1 ms).
High-resolution time resolved transmittivity measurements on horizontally aligned freestanding multi-walled carbon nanotubes reveal a different electronic transient behavior from that of graphite. This difference is ascribed to the presence of discrete energy states in the multishell carbon nanotube electronic structure. Probe polarization dependence suggests that the optical transitions involve definite selection rules. The origin of these states is discussed and a rate equation model is proposed to rationalize our findings.
In this letter we report a facile one-pot synthesis of intercalated ZnO particles for inexpensive, low-temperature solution processed dye-sensitised solar cells. High interconnectivity facilitates enhanced charge transfer between the ZnO nanoparticles and a consequent enhancement in cell efficiency. ZnO thin films were formed from a wide range of nanoparticle diameters which simultaneously increased optical scattering whilst enhancing dye loading. A possible growth mechanism was proposed for the synthesis of ZnO nanoparticles. The intercalated ZnO nanoparticle thin films were integrated into the photoanodes of dye-sensitised solar cells which showed an increase in performance of 37% compared to structurally equivalent cells employing ZnO nanowires.
We present electronically controlled field emission characteristics of arrays of individually ballasted carbon nanotubes synthesized by plasma-enhanced chemical vapor deposition on silicon-on-insulator substrates. By adjusting the source-drain potential we have demonstrated the ability to controllable limit the emission current density by more than 1 order of magnitude. Dynamic control over both the turn-on electric field and field enhancement factor have been noted. A hot electron model is presented. The ballasted nanotubes are populated with hot electrons due to the highly crystalline Si channel and the high local electric field at the nanotube base. This positively shifts the Fermi level and results in a broad energy distribution about this mean, compared to the narrow spread, lower energy thermalized electron population in standard metallic emitters. The proposed vertically aligned carbon nanotube field-emitting electron source offers a viable platform for X-ray emitters and displays applications that require accurate and highly stable control over the emission characteristics.
We report on an inexpensive, facile and industry viable carbon nanofibre catalyst activation process achieved by exposing stainless steel mesh to an electrolyzed metal etchant. The surface evolution of the catalyst islands combines low-rate electroplating and substrate dissolution. The plasma enhanced chemical vapour deposited carbon nanofibres had aspect-ratios >150 and demonstrated excellent height and crystallographic uniformity with localised coverage. The nanofibres were well-aligned with spacing consistent with the field emission nearest neighbour electrostatic shielding criteria, without the need of any post-growth processing. Nanofibre inclusion significantly reduced the emission threshold field from 4.5 V/μm (native mesh) to 2.5 V/μm and increased the field enhancement factor to approximately 7000.
An improved technique for transferring large area graphene grown by chemical vapor deposition on copper is presented. It is based on mechanical separation of the graphene/copper by H2 bubbles during H2O electrolysis, which only takes a few tens of seconds while leaving the copper cathode intact. A semi-rigid plastic frame in combination with thin polymer layer span on graphene gives a convenient way of handling- and avoiding wrinkles and holes in graphene. Optical and electrical characterizations prove the graphene quality is better than that obtained by traditional wet etching transfer. This technique appears to be highly reproducible and cost efficient.
Direct formation of large-area carbon thin films on gallium nitride by chemical vapor deposition without metallic catalysts is demonstrated. A high flow of ammonia is used to stabilize the surface of the GaN (0001)/sapphire substrate during the deposition at 950°C. Various characterization methods verify that the synthesized thin films are largely sp2 bonded, macroscopically uniform, and electrically conducting. The carbon thin films possess optical transparencies comparable to that of exfoliated graphene. This paper offers a viable route toward the use of carbon-based materials for future transparent electrodes in III-nitride optoelectronics, such as GaN-based light emitting diodes and laser diodes.
Thin-film electronics in its myriad forms has underpinned much of the technological innovation in the fields of displays, sensors, and energy conversion over the past four decades. This technology also forms the basis of flexible electronics. Here we review the current status of flexible electronics and attempt to predict the future promise of these pervading technologies in healthcare, environmental monitoring, displays and human–machine interactivity, energy conversion, management and storage, and communication and wireless networks.
Herein we present an inexpensive facile wet-chemistry-free approach to the transfer of chemical vapour-deposited multiwalled carbon nanotubes to flexible transparent polymer substrates in a single-step process. By controlling the nanotube length, we demonstrate accurate control over the electrical conductivity and optical transparency of the transferred thin films. Uniaxial strains of up to 140% induced only minor reductions in sample conductivity, opening up a number of applications in stretchable electronics. Nanotube alignment offers enhanced functionality for applications such as polarisation selective electrodes and flexible supercapacitor substrates. A capacitance of 17μF/g was determined for supercapacitors fabricated from the reported dry-transferred MWCNTs with the corresponding cyclic voltagrams showing a clear dependence on nanotube length.
One-dimensional ferroelectric nanostructures, carbon nanotubes (CNT) and CNT–inorganic oxides have recently been studied due to their potential applications for microelectronics. Here, we report coating of a registered array of aligned multi-wall carbon nanotubes (MWCNT) grown on silicon substrates by functional ferroelectric Pb(Zr, Ti)O3 (PZT) which produces structures suitable for commercial prototype memories. Microstructural analysis reveals the crystalline nature of PZT with small nanocrystals aligned in different directions. First-order Raman modes of MWCNT and PZT/MWCNT/n-Si show the high structural quality of CNT before and after PZT deposition at elevated temperature. PZT exists mostly in the monoclinic Cc/Cm phase, which is the origin of the high piezoelectric response in the system. Low–loss square piezoelectric hysteresis obtained for the 3D bottom-up structure confirms the switchability of the device. Current–voltage mapping of the device by conducting atomic force microscopy (c-AFM) indicates very low transient current. Fabrication and functional properties of these hybrid ferroelectric–carbon nanotubes is the first step towards miniaturization for future nanotechnology sensors, actuators, transducers and memory devices.
Metal-catalyst-free chemical vapor deposition(CVD) of large area uniform nanocrystallinegraphene on oxidized silicon substrates is demonstrated. The materialgrows slowly, allowing for thickness control down to monolayergraphene. The as-grown thin films are continuous with no observable pinholes, and are smooth and uniform across whole wafers, as inspected by optical-, scanning electron-, and atomic force microscopy. The sp2 hybridized carbon structure is confirmed by Raman spectroscopy. Room temperature electrical measurements show ohmic behavior (sheet resistance similar to exfoliated graphene) and up to 13% of electric-field effect. The Hall mobility is 40 cm2/Vs, which is an order of magnitude higher than previously reported values for nanocrystalline graphene.Transmission electron microscopy, Raman spectroscopy, and transport measurements indicate a graphene crystalline domain size 10 nm. The absence of transfer to another substrate allows avoidance of wrinkles, holes, and etching residues which are usually detrimental to device performance. This work provides a broader perspective of graphene CVD and shows a viable route toward applications involving transparent electrodes.
A noncatalytic chemical vapor deposition mechanism is proposed, where high precursor concentration, long deposition time, high temperature, and flat substrate are needed to grow large-area nanocrystalline graphene using hydrocarbon pyrolysis. The graphene is scalable, uniform, and with controlled thickness. It can be deposited on virtually any nonmetallic substrate that withstands 1000°C. For typical examples, graphenegrown directly on quartz and sapphire shows transmittance and conductivity similar to exfoliated or metal-catalyzed graphene, as evidenced by transmission spectroscopy and transport measurements. Raman spectroscopy confirms the sp2-C structure. The model and results demonstrate a promising transfer-free technique for transparent electrode production.
We present our observations made during the early stages of graphene growth employing an ethylene-based CVD method capable of synthesizing copper-foil-catalyzed monolayer graphene at temperatures as low as 800°C. Spectroscopic monitoring of surface catalysis showed that graphene crystals evolve from densely distributed nucleation points that interconnect to form large crystals covering the entire surface. Secondary nucleation was observed inside the primary graphene crystals. An effective activation energy for copper-catalyzed ethylene-based graphene synthesis was determined to be 2.46 eV, a value that suggests surface dehydrogenation of ethylene or lattice integration of graphene as the possible rate-determining step in the heterogeneous catalysis.
A systematic study of the Cu-catalyzed chemical vapor deposition of graphene under extremely low partial pressure is carried out. A carbon precursor supply of just PCH4 ~ 0.009 mbar during the deposition favors the formation of large-area uniform monolayer graphene verified by Raman spectra. A diluted HNO3 solution is used to remove Cu before transferring graphene onto SiO2/Si substrates or carbon grids. The graphene can be made suspended over a ~12 μm distance, indicating its good mechanical properties. Electron transport measurements show the graphene sheet resistance of ~0.6 kΩ/sq. at zero gate voltage. The mobilities of electrons and holes are ~1800 cm2/Vs at 4.2 K and ~1200 cm2/Vs at room temperature.
Large-area uniform carbon films with graphene-like properties are synthesized by chemical vapor deposition directly on Si3N4/Si at 1000°C without metal catalysts. The as deposited films are atomically thin and wrinkle- and pinhole-free. The film thickness can be controlled by modifying the growth conditions. Raman spectroscopy confirms the sp2 graphitic structures. The films show ohmic behavior with a sheet resistance of 2.3– 10.5 kΩ/sq. at room temperature. An electric field effect of 2–10% VG =−20 V is observed. The growth is explained by the self-assembly of carbon clusters from hydrocarbon pyrolysis. The scalable and transfer-free technique favors the application of graphene as transparent electrodes.
We report on a quantum dot sensitized solar cell (QDSSC) based on ZnO nanorod coated vertically aligned carbon nanotubes (VACNTs). Electrochemical impedance spectroscopy shows that the electron lifetime for the device based on VACNT/ZnO/CdSe is longer than that for a device based on ZnO/CdSe, indicating that the charge recombination at the interface is reduced by the presence of the VACNTs. Due to the increased surface area and longer electron lifetime, a power conversion efficiency of 1.46% is achieved for the VACNT/ZnO/CdSe devices under an illumination of one Sun (AM 1.5G, 100 mW/cm2).
With many thousands of different varieties to date, the nanowire (NW) library continues to grow at pace. With the continued and hastened maturity of nanotechnology, significant advances in materials science have allowed for the rational synthesis of a myriad of NW types of unique electronic and optical properties, allowing for the realisation of a wealth of novel devices, whose use is touted to become increasingly central in a number of emerging technologies. Nanowires, structures defined as having diameters between 1 and 100 nm, provide length scales at which a variety of inherent and unique physical effects come to the fore , phenomena which are often size suppressed in their bulk counterparts [2–4]. It is these size- dependent effects that have underpinned the growing interest in the growth and fabrication, at ever more commercial scales, of nanoscale structures. Nevertheless, many of the intrinsic properties of such NWs become largely smeared and often entirely lost, when they adopt disordered ensembles. Conversely, ordered and aligned NWs have been shown to retain many such properties, alongside proffering various new properties that manifest on the micro- and even macroscale that would hitherto not occur in their disordered counterparts.
This six-volume handbook focuses on fabrication methods, nanostructure and atomic arrangement, electrical and optical properties, mechanical and chemical properties, size-dependent properties, and applications and industrialization. There is no other major reference work of this scope on the topic of graphene, which is one of the most researched materials of the twenty-first century. The set includes contributions from top researchers in the field and a foreword written by two Nobel laureates in physics.
This six-volume handbook focuses on fabrication methods, nanostructure and atomic arrangement, electrical and optical properties, mechanical and chemical properties, size-dependent properties, and applications and industrialization. There is no other major reference work of this scope on the topic of graphene, which is one of the most researched materials of the twenty-first century. The set includes contributions from top researchers in the field and a foreword written by two Nobel laureates in physics.
A comprehensive and practical analysis and overview of the imaging chain through acquisition, processing and display The Handbook of Digital Imaging provides a coherent overview of the imaging science amalgam, focusing on the capture, storage and display of images. The volumes are arranged thematically to provide a seamless analysis of the imaging chain from source (image acquisition) to destination (image print/display). The coverage is planned to have a very practical orientation to provide a comprehensive source of information for practicing engineers designing and developing modern digital imaging systems. The content will be drawn from all aspects of digital imaging including optics, sensors, quality, control, colour encoding and decoding, compression, projection and display.
Carbon-based nanomaterials are rapidly emerging as one of the most fascinating materials in the twenty-first century. Chemical Functionalization of Carbon Nanomaterials: Chemistry and Applications provides a thorough examination of carbon nanomaterials, including their variants and how they can be chemically functionalized. It also gives a comprehensive overview of current advanced applications of functionalized carbon nanomaterials, including the automotive, packaging, coating, and biomedical industries.
The book covers modern techniques to characterize chemically functionalized carbon nanomaterials as well as characterization of surface functional groups. It includes contributions from international leaders in the field who highlight the multidisciplinary and interdisciplinary flexibility of functionalized carbon nanomaterials. The book illustrates how natural drawbacks to carbon nanomaterials, such as low solubility, can be countered by surface modifications and shows how to make modifications.
It discusses developments in the use of carbon nanomaterials in several critical areas in scientific research and practice, including analytical chemistry, drug delivery, and water treatment. It explores market opportunities due to the versatility and increasing applicability of carbon nanomaterials. It also gives suggestions on the direction of the field from its current point, paving the way for future developments and finding new applications. Chemical Functionalization of Carbon Nanomaterials: Chemistry and Applications is a significant collection of findings in a rapidly developing field. It gives an in-depth look at the current achievements of research and practice while pointing you ahead to new possibilities in functionalizing and using carbon nanomaterials.
In the second edition of Emerging Nanotechnologies for Manufacturing, an unrivalled team of international experts explores existing and emerging nanotechnologies as they transform large-scale manufacturing contexts in key sectors such as medicine, advanced materials, energy, and electronics. From their different perspectives, the contributors explore technologies and techniques as well as applications and how they transform those sectors.
With updated chapters and expanded coverage, the new edition of Emerging Nanotechnologies for Manufacturing reflects the latest developments in nanotechnologies for manufacturing and covers additional nanotechnologies applied in the medical fields, such as drug delivery systems. New chapters on graphene and smart precursors for novel nanomaterials are also added. This important and in-depth guide will benefit a broad readership, from R&D scientists and engineers to venture capitalists.
Carbon, though often overlooked as an engineering material by many, underpins much infrastructure central to our modern society. The recent dawn of the nano-age has solidified Carbons engineering importance. Once the exclusive preserve of the laboratory worker, carbon-based nanotechnology is beginning to make the lab-to-fab transition.
More than three decades have passed since the discovery of the C60 Buckminster Fullerene. Heralding the start of the nanocarbon age, the field has developed rapidly since with a diverse set of tools being realised that are capable of isolating, synthesising and integrating the graphitic nanocarbons; namely, the fullerenes, carbon nanotubes and, more recently, graphene. As a result, the number of ensuing applications based on these materials has continued to expand; from advanced composites, to unique computational elements, the list continues to grow, in line with the increasing inventiveness of the nanocarbon community. It is carbon’s allotropic diversity, and potential for ready augmentation through chemical functionalisation that has, in part, resulted in the wide potential applications of these materials. By 2015 there were 114,000 indexed publications on carbon nanotechnology alone, with carbon nanotubes and graphene accounting for approximately 76% of these. This proportion continues to grow aggressively. From 2000 to 2010, carbon nanotubes dominated the carbon based research environment, however; in 2013 the number of annual publications of graphene surpassed those of carbon nanotubes. Developments in carbon nanotechnology, and in particular the carbon nanotubes and the graphenes, show no indication of slowing.
The field of carbon nanotechnology continues to mature, becoming ever more interdisciplinary and collaborative. It is this breadth that necessitates timely summary works providing the wider readership with accessible content that represents some of the most recent developments. Written; by international leading figures within the research community, and for academics and industrial professionals alike, Carbon Nanotechnology is structured to provide the advanced reader with a succinct point of reference, and those readers new to the field, a tractable introduction to carbon nanotechnology and a foundation onto which they may continue to build their interest. This text does not document incremental advances in the field; such cataloguing would be prohibitive. Rather, we distil some of the key recent developments, primarily as it relates to carbon nanotubes, and to a lesser extent, given its relative infancy, graphene. The primary sections of the book centre on the growth, characterisation, and integration of carbon nanotubes, with the latter section focussing on graphene growth and its micro-structured derivatives. We have intentionally neglected to include advances in Fullerene technology given the shear breadth of work on this that has come before.
We hope this collection of articles serves you, the reader, as a concise and useful summary of the state-of-the-art in carbon nanotechnology, and that it functions as a useful referencing tool to highlight the various outstanding challenges to help guide future research.
Commercial interest in graphene is increasing, and at a pace. The concurrently high mechanical robustness and electrical conductance make the material suitable for use in a wide array of emerging flexible electronics devices. Theoretically, graphene is stronger than diamond, more conductive than copper, and more flexible than rubber - it has much potential for use in a myriad of flexible electronics applications. However, upon transfer, monolayer graphene has been reported to have poor mechanical robustness and low electrical conductivity relative to theoretical estimates. This has limited the commercial adoption. This thesis presents the author’s efforts towards the development of two new graphene transfer methods; Hot Press Laminate (HPL), and Ultra Violet Adhesive (UVA) transfer, both of which have been shown to enhance the degree of adhesion between the as-grown graphene and the polymer carrier substrate. To assess the feasibility of the transfer approach, sheet resistance (RS) maps and optical transmittance (%T) maps of the transferred graphene using the two approaches have been measured, alongside a number of metrological studies to better understand the means though which the adhesion is improved and the effects such improved adhesion has on the transport within these two-dimensional thin films. The adhesion of transferred graphene on a substrate has been examined using conventional peel-off tests and the mechanical robustness of the transferred graphene was investigated by measuring resistance as a function of bend angle with repeated bend-relax cycles in a fully-automated, custom-built bend system. Because of its transparency and high conductivity graphene is especially useful in applications requiring transparent contacts, such as in organic light emitting diodes (OLEDs). To further increase the electrical conductivity and tune the work function of the as-grown graphene for such applications, chemical doping has been investigated using common metal chloride compounds; AuCl3, FeCl3, SnCl2, IrCl3 and RhCl3. The spatial and temporal variation in RS, %T, and contact angle have been measured for nominally pristine and doped graphene samples. Micron-scale spatial mapping of the conductivity has also been conducted revealing edge-mediated conduction in doped graphene. It was determined that the high electrical conductivity of the doped graphene was principally due to charge transfer from the dopant to the graphene, with such charge transfer resulting in a notable work function shift. This work function shift, a critical parameter for various systems such as field electron emitters and OLEDs, was independently quantified using Ultra-violet Photo-Spectroscopy (UPS), Kelvin Prove Force Microscopy (KPFM), Hall measurement, and Density Functional Theory (DFT) and was subsequently compared to the Gibb’s free energy and standard reduction potential of the composite metal ions within the metal chlorides. Temperaturedependent transport studies (I-V) suggested that the Fermi level shift of doped graphene enhanced charge carrier transport by increasing the transmission coefficient associated with intrinsic potential barriers within the graphene, such as the carbon atoms and grain boundaries. Variable Range and Nearest Neighbour Hopping were found to dominate the transport in the undoped graphene, whilst transport in doped graphene is principally attributed to combined Nearest Neighbour Hopping and diffusive transport.
In the quest for reliable, repeatable and stable field electron emission that has commercial potential, whilst many attempts have been made, none yet has been truly distinguishable as being successful. Whilst I do not claim within this thesis to have uncovered the secret to success, fundamental issues have been addressed that concern the future directions towards achieving its full potential. An exhaustive comparison is made across the diverse range of materials that have, over the past 40-50 years, been postulated and indeed tested as field emitters. This has not previously been attempted. The materials are assessed according to the important metrics of turn on voltage, Eon, and maximum current density, Jmax, where low Eon and high Jmax are seen as desirable. The nano-carbons, carbon nanotubes (CNTs), in particular, perform well in both these metrics. No dependency was seen between the material work function and its performance as an emitter, which might have been suggested by the Fowler Nordheim equations. To address the issues underlying the definition of the local enhancement factor, β, a number of variations of surface geometry using CNTs were fabricated. The field emission of these emitters was measured using two different approaches. The first is a Scanning Electrode Field Emission Microscope, SAFEM, which maps the emission at individual locations across the surface of the emitter, and the parallel plate that is more commonly encountered in field emission measurements. Finally, an observed hysteretic behaviour in CNT field emission was explored. The field emitters were subjected to a number of tests. These included; in-situ residual gas analysis of the gas species in the emitter environment, a stability study in which the emitters were exposed to a continuing voltage loop for 50 cycles, differing applied voltage times to analyse the effects on the emitted current, and varying maximums of applied field in a search for hysteresis onset information. These studies revealed the candidate in causing the hysteresis is likely to be water vapour that adsorbs on the CNT surface. A six step model if the emission process was made that details how and when the hysteresis is caused.