Kazakova O, Gallop JC, See P, Cox D, Perkins GK, Moore JD, Cohen LF (2009) Detection of a Micron-Sized Magnetic Particle Using InSb Hall Sensor, IEEE TRANSACTIONS ON MAGNETICS 45 (10) pp. 4499-4502 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Shiozawa H, Bachmatiuk A, Stangl A, Cox DC, Silva SRP, Ruemmeli MH, Pichler T (2013) Microscopic insight into the bilateral formation of carbon spirals from a symmetric iron core, SCIENTIFIC REPORTS 3 ARTN 1840 NATURE PUBLISHING GROUP
Bechstein S, Ruede F, Drung D, Storm J-H, Koehn C, Kieler OF, Kohlmann J, Weimann T, Patel T, Li B, Cox D, Gallop JC, Hao L, Schurig T (2015) Design and Fabrication of Coupled NanoSQUIDs and NEMS,IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 25 (3) ARTN 16026
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
© 2014 IEEE.NanoSQUIDs made from Nb thin films have been produced with nanometre loop sizes down to 200 nm, using weak-link junctions with dimensions less than 60 nm. These composite (W/Nb) single layer thin film devices, patterned by FIB milling, show extremely good low-noise performance ~170 n¦0 at temperatures between 5 and 8.5 K and can operate in rather high magnetic fields (at least up to 1 T). The devices produced so far have a limited operating temperature range, typically only 1-2 K. We have the goal of achieving operation at 4.2 K, to be compatible with the best SQUID series array (SSA) preamplifier available. Using the SSA to readout the nanoSQUIDs provides us with a means of investigating the intrinsic noise of the former. In this paper we report improved white noise levels of these nanoSQUIDs, enabling potential detection of a single electronic spin flip in a 1-Hz bandwidth. At low frequencies the noise performance is already limited by SSA preamplifier noise.
Langridge MT, Cox DC, Webb RP, Stolojan V (2014) The fabrication of aspherical microlenses using focused ion-beam techniques, Micron 57 (C) pp. 56-66
© 2013 Elsevier Ltd.Aspheric lenses are the most common method for correcting for spherical aberrations but, in microlens production, highly-controlled lens profiles are hard to achieve. We demonstrate a technique for creating bespoke, highly-accurate aspheric or spherical profile silicon microlens moulds, of almost any footprint, using focused ion-beam milling. Along with this, we present a method of removing induced ion-beam damage in silicon, via a hydrofluoric acid etch, helping to recover the surface's optical and chemical properties. In this paper, we demonstrate that our milled and etched moulds have a roughness of 4.0-4.1. nm, meaning they scatter less than 1% of light, down to wavelengths of 51. nm, showing that the moulds are suitable to make lenses that are able to handle light from UV up to infra-red.Using empirical experiments and computer simulations, we show that increasing the ion-dose when milling increases the amount of gallium a hydrofluoric acid etch can remove, by increasing the degree of amorphisation within the surface. For doses above 3000¼C/cm2 this restores previous surface properties, reducing adhesion to the mould, allowing for a cleaner release and enabling higher quality lenses to be made.Our technique is used to make aspheric microlenses of down to 3. ¼m in size, but with a potential to make lenses smaller than 1. ¼m.
Electron beam deposition of tungsten from W(CO)6 has been studied for a range of different height deposits. The resistance of the deposited tracks was found to decrease with increasing height implying that thicker deposits have a higher metallic cross section. It was also found that when the current limits were increased to values in excess of 100 ¼A, the resistance decreased with successive voltage cycles and increasing current limit. This improvement continued until a point where the structure of the deposit appeared to start breaking down and its resistance increased slightly. Subsequently, the structure was found to break near to the contact. It was also found that as the resistance of the deposit decreased, the structure of the deposited tracks changed implying that ohmic heating induced high enough temperatures within the deposit to be able to cause the structure of the material to change. © 2008 IOP Publishing Ltd.
As metrology extends toward the nanoscale, a number of potential applications and new challenges arise. By combining photolithography with focused ion beam and/or electron beam methods, superconducting quantum interference devices (SQUIDs) with loop dimensions down to 200 nm and superconducting bridge dimensions of the order 80 nm have been produced. These SQUIDs have a range of potential applications. As an illustration, we describe a method for characterizing the effective area and the magnetic penetration depth of a structured superconducting thin film in the extreme limit, where the superconducting penetration depth » is much greater than the film thickness and is comparable with the lateral dimensions of the device. © 2007 IEEE.
Miyajima Y, Shannon JM, Henley SJ, Stolejan V, Cox DC, Silva SRP (2007) Electrical conduction mechanism in laser deposited amorphous carbon, THIN SOLID FILMS 516 (2-4) pp. 257-261 ELSEVIER SCIENCE SA
Tison V, Stolojan V, Watts PCP, Cox DC, Chen GY, Silva SRP (2008) Gas Sensing Properties of Vapour-Deposited Tungsten Oxide Nanostructures, MICROSCOPY OF SEMICONDUCTING MATERIALS 2007 120 pp. 281-284 SPRINGER-VERLAG BERLIN
Rozhko S, Hino T, Blois A, Hao L, Gallop JC, Cox DC, Romans EJ (2013) Study of Low-Frequency Noise Performance of Nanobridge-Based SQUIDs in External Magnetic Fields, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 23 (3) ARTN 1601004 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Hao L, Cox D, Lees K, Gallop JC, See P, Clarke R, Janssen TJBM, Zhang RF, Wei F (2012) Fabrication and Characterization of Carbon Nanotubes as r.f. Interconnects, 2012 12TH IEEE CONFERENCE ON NANOTECHNOLOGY (IEEE-NANO) IEEE
Cox D, Kazakova O, Bowyer E, Favata A-L, Tzalenchuk A (2009) Superconducting Nanostructures in a Single Step Process, 25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PT 5A 150 (5a)
Smith PR, Carey JD, Cox DC, Forrest RD, Silva SRP (2009) On the importance of the electrostatic environment for the transport properties of freestanding multiwall carbon nanotubes, NANOTECHNOLOGY 20 (14) ARTN 145202 IOP PUBLISHING LTD
Renna F, Brambilla G, Cox DC (2009) Light Confinement in Optical Fibers Using Surface Plasmon Polaritons, IEEE PHOTONICS TECHNOLOGY LETTERS 21 (20) pp. 1508-1510 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Thomson DJ, Gardes FY, Cox DC, Fedeli JM, Mashanovich GZ, Reed GT (2013) Self-aligned silicon ring resonator optical modulator with focused ion beam error correction, Journal of the Optical Society of America B: Optical Physics 30 (2) pp. 445-449
Ring-resonator-based silicon optical modulators have tremendous potential for forming compact and power-efficient optical transmitters. In this paper we demonstrate high-speed modulation from a device based upon carrier depletion in a self-aligned pn junction. Furthermore, the role that focused ion beams can play in the error correction of silicon photonic devices is also demonstrated. © 2013 Optical Society of America.
Rod I, Wirtz C, Kazakova O, Panchal V, Cox D, Zähres H, Posth O, Lindner J, Meckenstock R, Farle M (2012) Carbon nanotube bolometer: Transport properties and noise characteristics,Solid State Phenomena 190 pp. 510-513
The temperature dependent resistance and the noise characteristics of an individual multiwall carbon nanotube (CNT) decorated with a finite number of magnetic nanocubes are investigated. We show that CNT is a highly sensitive bolometer and can enable measurements of magnetic resonance in a single nanoparticle. © (2012) Trans Tech Publications.
Zhang W, Shiozawa H, Wu CW, Hamerton I, Cox DC, Silva SR (2012) The effect of pH on the functionalization of nylon fabric with carbon nanotubes., J Nanosci Nanotechnol 12 (1) pp. 84-90
Single walled carbon nanotubes (SWCNTs) were dispersed in water and attached to nylon fabrics by a dip-drying procedure; scanning electron microscopy and Raman spectroscopy suggest the attachment of the SWCNTs. The electrical resistance of the functionalized fabrics is found to be pH-dependent, which is correlated with the quantity of SWCNTs dispersed in water at different values of pH. This can be further ascribed to the influence of the pK(a) of the acid (e.g., acetic acid in this study) used to tune pH. The acid may affect the dispersion of SWCNTs through two different mechanisms: (1) the free protons may protonate the amine and/or sulfonate group in the dye molecules, resulting in a variety of interactions among the dye molecules, SWCNTs and water molecules and (2) the resulting ions may increase the ionic strength of the solution, compressing the electric double layers of SWCNT colloids and thus impairing their stability. The former possibility is ruled out by data obtained using X-ray photoelectron spectroscopy, Raman spectroscopy, and ultraviolet-visible-near infrared spectroscopy; thus the latter is proposed to account for the experimental results. The colour strength of the functionalized fabrics increases with increasing pH, which is in agreement with their measured electrical properties.
Hao L, Cox D, See P, Gallop J, Kazakova O (2010) Magnetic nanoparticle detection using nano-SQUID sensors, JOURNAL OF PHYSICS D-APPLIED PHYSICS 43 (47) ARTN 474004 IOP PUBLISHING LTD
Light confinement to sub-wavelength spot sizes is proposed and realized in tapered optical fibers. To achieve high transmission efficiencies, light propagating along the taper is combined with the excitation of surface plasmon polaritons (SPP) at its tip. (C) 2009 Optical Society of America
Curry RJ, cox dc, devarepally kk, fry AT, stolojan, munz m (2011) Synthesis of linear ZnO structures by a
thermal decomposition method and their
characterisation, Journal of Materials Science 47 pp. 1893-1901
The semiconductor zinc oxide (ZnO) is a
promising material for applications in optoelectronics, photochemistry and chemical sensing. Furthermore, ZnO structures can be grown with a large variety of sizes and shapes. Devices with ZnO rods or wires as their core elements can be used in solar cells, gas sensors or biosensors. In this article, an easy approach for the non-aqueous wet chemical synthesis of ZnO structures is presented that employs the solvent trioctylamine (TOA) and the surfactant hexamethylenetetramine (HMTA). Using the thermal decomposition method, rod-shaped structures were grown that are suitable for the fabrication of electrical devices. A detailed study was carried out to investigate the effects of various reaction parameters on the growth process. Both the concentration of the surfactant HMTA and the zinc precursor zincacetylacetonate (Zn(acac)2) were found to show strong effects on the resulting morphology. In addition to structural characterisation using XRD, SEM and TEM, also optical properties of rod-shaped ZnO structures were measured. Rod-shaped structures were obtained for the following conditions: reaction time 4 h,reaction temperature 70 C, 1 mmol of Zn(acac)2, 4 mmol of HMTA and 25 mL of the solvent TOA. Photoluminescence and photoluminescence excitation spectroscopy of samples grown under these conditions provided information on levels of defect states that could be critical for chemical sensing applications. Two narrow peaks around 254 and 264 nm were found that are well above the band gap of ZnO.
Palumbo M, Henley SJ, Lutz T, Stolojan V, Cox D, Silva SRP (2008) Engineering the shape of zinc oxide crystals via sonochemical or hydrothermal solution-based methods, Materials Research Society Symposium Proceedings 1087 pp. 59-65
Recent results in the use of Zinc Oxide (ZnO) nano/submicron crystals in fields as diverse as sensors, UV lasers, solar cells, piezoelectric nanogenerators and light emitting devices have reinvigorated the interest of the scientific community in this material. To fully exploit the wide range of properties offered by ZnO, a good understanding of the crystal growth mechanism and related defects chemistry is necessary. However, a full picture of the interrelation between defects, processing and properties has not yet been completed, especially for the ZnO nanostructures that are now being synthesized. Furthermore, achieving good control in the shape of the crystal is also a very desirable feature based on the strong correlation there is between shape and properties in nanoscale materials. In this paper, the synthesis of ZnO nanostructures via two alternative aqueous solution methods - sonochemical and hydrothermal - will be presented, together with the influence that the addition of citric anions or variations in the concentration of the initial reactants have on the ZnO crystals shape. Foreseen applications might be in the field of sensors, transparent conductors and large area electronics possibly via ink-jet printing techniques or self-assembly methods. © 2008 Materials Research Society.
© 2015 AIP Publishing LLC.We investigate the effect of a single magnetic bead (MB) on the domain wall (DW) pinning/depinning fields of a DW trapped at the corner of an L-shaped magnetic nanodevice. DW propagation across the device is investigated using magnetoresistance measurements. DW pinning/depinning fields are characterized in as-prepared devices and after placement of a 1 ¼m-sized MB (Dynabeads® MyOne") at the corner. The effect of the MB on the DW dynamics is seen as an increase in the depinning field for specific orientations of the device with respect to the external magnetic field. The shift of the depinning field, Bdep = 4.5-27.0 mT, is highly stable and reproducible, being significantly above the stochastic deviation which is about 0.5 mT. The shift in the deppinning field is inversely proportional to the device width and larger for small negative angles between the device and the external magnetic field. Thus, we demonstrate that DW-based devices can be successfully used for detection of single micron size MB.
Gomez-Rojas L, Bhattacharyya S, Mendoza E, Cox DC, Rosolen JM, Silva SRP (2007) RF response of single-walled carbon nanotubes, NANO LETTERS 7 (9) pp. 2672-2675 AMER CHEMICAL SOC
Howe S, Headley WR, Cox DC, Mashanovich GZ, Thomson DJ, Reed GT (2009) Fabrication & Tailoring of Silicon Photonic Devices via Focused Ion Beam, SILICON PHOTONICS IV 7220 ARTN 722011 SPIE-INT SOC OPTICAL ENGINEERING
© 2015 Optical Society of America.We report on the fabrication and diffraction-limited characterization of parabolic focusing micromirrors. Sub-micron beam waists are measured for mirrors with 10-¼m radius aperture and measured fixed focal lengths in the range from 24 ¼m to 36 ¼m. Optical characterization of the 3D intensity in the near-field produced when the device is illuminated with collimated light is performed using a modified confocal microscope. Results are compared directly with angular spectrum simulations, yielding strong agreement between experiment and theory, and identifying the competition between diffraction and focusing in the regime probed.
Smith RC, Cox DC, Silva SRP (2008) In situ field emission characterization of multiwalled carbon nanotubes, JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 26 (2) pp. 842-846 A V S AMER INST PHYSICS
Rod I, Kazakova O, Cox DC, Spasova M, Farle M (2009) The route to single magnetic particle detection: a carbon nanotube decorated with a finite number of nanocubes, NANOTECHNOLOGY 20 (33) ARTN 335301 IOP PUBLISHING LTD
Burch HJ, Brown E, Contera SA, Toledo NC, Cox DC, Grobert N, Hao L, Ryan JF, Davies JA (2008) Effect of acid treatment on the structure and electrical properties of nitrogen-doped multiwalled carbon nanotubes, JOURNAL OF PHYSICAL CHEMISTRY C 112 (6) pp. 1908-1912 AMER CHEMICAL SOC
Hao L, Cox DC, Gallop JC (2009) Characteristics of focused ion beam nanoscale Josephson devices, SUPERCONDUCTOR SCIENCE & TECHNOLOGY 22 (6) ARTN 064011 IOP PUBLISHING LTD
Wain AJ, Cox D, Zhou S, Turnbull A (2011) High-aspect ratio needle probes for combined scanning electrochemical microscopy-Atomic force microscopy, ELECTROCHEMISTRY COMMUNICATIONS 13 (1) pp. 78-81 ELSEVIER SCIENCE INC
Miloaevi? MM, Thomson DJ, Chen X, Cox D, Mashanovich GZ (2011) Silicon waveguides for the 3-4 ¼m wavelength range, IEEE International Conference on Group IV Photonics GFP pp. 208-210
In this paper we report propagation and bend loss measurements for silicon-on-insulator (SOI) and silicon-on-sapphire (SOS) waveguides at 3.39 ¼m wavelength. Preliminary experimental results for SOI rib waveguides at around 3.8 ¼m are also given. © 2011 IEEE.
Jones HG, Mingard KP, Cox DC (2014) Investigation of slice thickness and shape milled by a focused ion beam for three-dimensional reconstruction of microstructures., Ultramicroscopy 139 pp. 20-28
Three-dimensional reconstructions of microstructures produced by focused ion beam (FIB) milling usually assume a uniform slice thickness with flat and parallel surfaces. Measurement of the actual slice thickness and profile is difficult, and is often simply ignored. This paper reports the use of artificial 3D structures of known geometry to enable the full 3D profile of a sequence of slices produced by FIB to be measured for the first time. A transient period at the beginning of a milling process is observed in which the actual slice thickness varies by as much as ±50% from the target thickness (with significantly greater error near the base of the slice), before settling to a ±20% variation as the milling progresses. Although SEM images appear to show flat milled surfaces perpendicular to the top surface, the development of a curved, tapering milled surface is also observed. This profile is then maintained through the milling process with the bottom of the slice lagging the top by up to three slice thicknesses.
Panchal V, Cox D, Yakimova R, Kazakova O (2013) Epitaxial Graphene Sensors for Detection of Small Magnetic Moments, IEEE TRANSACTIONS ON MAGNETICS 49 (1) pp. 97-100 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Shiozawa H, Skeldon AC, Lloyd DJ, Stolojan V, Cox DC, Silva SR (2011) Spontaneous emergence of long-range shape symmetry, Nano Letters 11 (1) pp. 160-163
Self-organization of matter is essential for natural pattern formation, chemical synthesis, as well as modern material science. Here we show that isovolumetric reactions of a single organometallic precursor allow symmetry breaking events from iron nuclei to the creation of different symmetric carbon structures: microspheres, nanotubes, and mirrored spiraling microcones. A mathematical model, based on mass conservation and chemical composition, quantitatively explains the shape growth. The genesis of such could have significant implications for material design.
Munz M, Langridge MT, Devarepally KK, Cox DC, Patel P, Martin NA, Vargha G, Stolojan V, White S, Curry RJ (2013) Facile synthesis of titania nanowires via a hot filament method and conductometric measurement of their response to hydrogen sulfide gas, ACS Applied Materials and Interfaces 5 (4) pp. 1197-1205
Titania nanostructures are of increasing interest for a variety of applications, including photovoltaics, water splitting, and chemical sensing. Because of the photocatalytical properties of TiO2, chemical processes that occur at its surface can be exploited for highly efficient nanodevices. A facile and fast synthesis route has been explored that is free of catalysts or templates. An environmental scanning electron microscopy (ESEM) system was employed to grow titania nanowires (NWs) in a water vapor atmosphere (
Thomson DJ, Gardes FY, Cox DC, Fedeli JM, Mashanovich GZ, Reed GT (2012) An alignment tolerant high speed ring resonator based silicon optical modulator, IEEE International Conference on Group IV Photonics GFP pp. 198-200
Optical modulation up to 40Gbit/s from a silicon ring resonator based device is demonstrated. A self-aligned process is used to form the pn junction reducing performance variations. The power consumption of the device is 8fJ/bit. © 2012 IEEE.
Webster CH, Giblin SP, Cox D, Janssen TJBM, Zorin AB (2008) A quantum current standard based on phase slip, 2008 CONFERENCE ON PRECISION ELECTROMAGNETIC MEASUREMENTS DIGEST pp. 628-629 IEEE
Smith RC, Carey JD, Cox DC, Silva SRP (2007) In-situ field emission characterisation of multi walled carbon nanotubes, 2007 IEEE 20TH INTERNATIONAL VACUUM NANOELECTRONICS CONFERENCE pp. 84-85 IEEE
Hao L, Cox DC, Gallop JC, Chen J, Rozhko S, Blois A, Romans EJ (2013) Coupled NanoSQUIDs and Nano-Electromechanical Systems (NEMS) Resonators, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 23 (3) ARTN 1800304 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Bechstein S, Kirste A, Drung D, Regin M, Kazakova O, Gallop J, Hao L, Cox D, Schurig T (2013) Investigation of Material Effects With Micro-Sized SQUID Sensors, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 23 (3) ARTN 1602004 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Reed GT, Thomson DJ, Gardes FY, Hu Y, Owens N, Debnath K, O'Faolain L, Krauss TF, Lever L, Ikonic Z, Kelsall RW, Myronov M, Leadley DR, Marko IP, Sweeney SJ, Cox DC, Brimont A, Sanchis P, Duan GH, Le Liepvre A, Jany C, Lamponi M, Make D, Lelarge F, Fedeli JM, Messaoudene S, Keyvaninia S, Roelkens G, Van Thourhout D, Liu S, Yang X, Petropoulos P (2012) High performance silicon optical modulators,Proceedings of SPIE - The International Society for Optical Engineering 8564
In this work we present results from high performance silicon optical modulators produced within the two largest silicon photonics projects in Europe; UK Silicon Photonics (UKSP) and HELIOS. Two conventional MZI based optical modulators featuring novel self-aligned fabrication processes are presented. The first is based in 400nm overlayer SOI and demonstrates 40Gbit/s modulation with the same extinction ratio for both TE and TM polarisations, which relaxes coupling requirements to the device. The second design is based in 220nm SOI and demonstrates 40Gbits/s modulation with a 10dB extinction ratio as well modulation at 50Gbit/s for the first time. A ring resonator based optical modulator, featuring FIB error correction is presented. 40Gbit/s, 32fJ/bit operation is also shown from this device which has a 6um radius. Further to this slow light enhancement of the modulation effect is demonstrated through the use of both convention photonic crystal structures and corrugated waveguides. Fabricated conventional photonic crystal modulators have shown an enhancement factor of 8 over the fast light case. The corrugated waveguide device shows modulation efficiency down to 0.45V.cm compared to 2.2V.cm in the fast light case. 40Gbit/s modulation is demonstrated with a 3dB modulation depth from this device. Novel photonic crystal based cavity modulators are also demonstrated which offer the potential for low fibre to fibre loss. In this case preliminary modulation results at 1Gbit/s are demonstrated. Ge/SiGe Stark effect devices operating at 1300nm are presented. Finally an integrated transmitter featuring a III-V source and MZI modulator operating at 10Gbit/s is presented. © 2012 SPIE.
Mashanovich GZ, Milosevic MM, Nedeljkovic M, Cox D, Passaro VMN, Chong HMH, Soref R (2012) Mid-infrared silicon photonics for sensing applications, Materials Research Society Symposium Proceedings 1437 pp. 7-16
The mid-infrared wavelength region offers a plethora of possible applications ranging from sensing, medical diagnostics and free space communications, to thermal imaging and IR countermeasures. Hence group IV mid-infrared photonics is attracting more research interest lately. Sensing is an especially attractive area as fundamental vibrations of many important gases are found in the 3 to 14 ¼m spectral region. To realise group IV photonic mid-infrared sensors several serious challenges need to be overcome. The first challenge is to find suitable material platforms for the mid-infrared. In this paper we present experimental results for passive mid-infrared photonic devices realised in silicon-on-insulator (SOI), silicon-on-sapphire (SOS), and silicon on porous silicon (SiPSi). Although silicon dioxide is lossy in most parts of the mid-infrared, we have shown that it has potential to be used in the 3-4 ¼m region. We have characterized SOI waveguides with
Kazakova O, Gallop JC, Cox DC, Brown E, Cuenat A, Suzuki K (2008) Optimization of 2DEG InAs/GaSb Hall Sensors for Single Particle Detection, IEEE TRANSACTIONS ON MAGNETICS 44 (11) pp. 4480-4483 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Munz M, Cox DC, Cumpson PJ (2008) Nano-scale shear mode testing of the adhesion of nanoparticles to a surface-support, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE 205 (6) pp. 1424-1428 WILEY-BLACKWELL
Koukharenko E, Li X, Nandhakumar I, Frety N, Beeby SP, Cox D, Tudor MJ, Schiedt B, Trautmann C, Bertsch A, White NM (2008) Towards a nanostructured thermoelectric generator using ion-track lithography, JOURNAL OF MICROMECHANICS AND MICROENGINEERING 18 (10) IOP PUBLISHING LTD
Romans EJ, Rozhko S, Young L, Blois A, Hao L, Cox D, Gallop JC (2011) Noise Performance of Niobium Nano-SQUIDs in Applied Magnetic Fields, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 21 (3) pp. 404-407 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
Lyth SM, Filip LD, Cox DC, Silva SRP (2007) Novel carbon nanotube based three terminal devices, 2007 IEEE 20TH INTERNATIONAL VACUUM NANOELECTRONICS CONFERENCE pp. 214-215 IEEE
Rajkumar RK, Manzin A, Cox DC, Silva SRP, Tzalenchuk A, Kazakova O (2013) 3-D mapping of sensitivity of graphene hall devices to local magnetic and electrical fields, IEEE Transactions on Magnetics 49 (7) pp. 3445-3448
We report the response of sub-micron epitaxial graphene Hall devices to localized and inhomogeneous magnetic field produced by an MFM tip. We analyze the magneto-transport properties of epitaxial graphene and report the independent contribution of magnetic and electric fields in the measured transverse voltage maps with respect to the lateral and vertical position of the tip. A finite element model has also been developed to support the experimental results. © 2013 IEEE.
Electron field emission from an isolated carbon nanotube (CNT) was performed in situ in a modified scanning electron microscope, over a range of anode to CNT tip separations, D, of 1-60 mu m. The threshold field required for an emission current of 100 nA was seen to decrease from a value of 42 V mu m(-1) at an anode to CNT tip separation of 1 mu m, asymptotically, to approach 4 V mu m(-1) at a separation of 60 mu m. It is proposed that at low D, the electric field enhancement factor (beta) reduces as the anode electrode approaches the CNT mimicking a parallel plate configuration. Under "far field" conditions, where D > 3 h, where h is the CNT height, the CNT enhancement factor is no longer dependant on D, as shown by the asymptotic behavior of the threshold field, and is purely a factor of the CNT height and radius. For each CNT to tip separation, measured emission current data together with the threshold field and enhancement, are consistent with a Fowler-Nordheim analysis for the far field conditions, and dispels the need for a novel emission mechanism to explain the results as has been proposed recently. (c) 2005 American Institute of Physics.
Tungsten oxide nanowires are grown directly on tungsten wires and plates using thermal heating in an acetylene and nitrogen mixture. By heating the tungsten in nitrogen ambient, single crystal tungsten oxide nanowires can be synthesized via a self-assembly mechanism. It was found that the yield can be significantly increased with the addition of acetylene, which also results in thinner nanowires, as compared to nanowires synthesized in an oxidizing ambient. The tungsten oxide nanowires are 5 to 15 nm in diameter and hundreds of nanometers in length. In some cases, the use of acetylene and nitrogen process gas would result in tungsten oxide nanowires samples that appear visually,transparent. Comparison of the growth using the acetylene/nitrogen or then air/nitrogen mixtures is carried out. A possible synthesis mechanism, taking into account the effect of hydrocarbon addition is proposed.
Despite the "darker than black" association attributed to carbon nanotube forests, here is shown that it is also possible to grow these structures, over heat-sensitive substrates, featuring highly transmissive characteristics from the UV to infrared wavelengths, for forest heights as high as 20 ¼m. The optical transmission is interpreted in terms of light propagation along channels that are self-generated by localized bundling of tubes, acting as waveguides. A good correlation is shown between the distribution of diameter sizes of these sub-wavelength voids and the transmission spectrum of the forests. For the shorter visible and near-UV wavelengths, this model shows that light propagates by channeling along individual vertical voids in the forests, which elucidates the origin for the widely-reported near-zero reflectance values observed in forests. For the longer infrared wavelengths, the mode spreads over many nanotubes and voids, and propagates along a "homogeneous effective medium". The strong absorption of the forest at the shorter wavelengths is correlated in terms of the stronger attenuation inside a waveguide cavity, according to the » attenuation dependency of standard waveguide theory. The realization of this material can lead to novel avenues in new optoelectronic device design, where the carbon nanotube forests can be used as highly conducting "scaffolds" for optically active materials, whilst also allowing light to penetrate to significant depths into the structure, in excess of 20 ¼m, enabling optical functionality. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
In this thesis we discuss the manufacture and characterisation of micro-optical elements, for guiding light into sub-wavelength beams & spots, and for use in super-resolution imaging.
A physical limit exists in microscopy where it is impossible to view object smaller than half the illuminating wavelength, via conventional means. In white light microscopy this creates an resolution limit of 321nm (at a wavelength of 500nm, in air). This places a limit on the smallest objects a researcher can study using optical microscopy.
We present a method for fabricating plano-convex lenses which, when placed in near proximity to the samples, boost magnification of conventional microscopes by up-to 2.5x and resolve features below 200nm, with white light illumination.
We also demonstrate a curved axicon Bessel-beam former, that produces long (17 micrometer) non-diffracting beams of light, that can be sub-wavelength in width, down to 2/3rds the wavelength.
In this thesis we contribute the following to current knowledge:
We describe a focused ion-beam milling technique to form bespoke geometry of parabolic & spherical curvature, including reflective dishes, of diameter 1-10 microns, with a surface roughness of 4.0-4.1nm.
As part of this work, we calculate the efficiency of a new technique for removing ion-beam induced damage, using wet-chemical etching. Here we show that increasing the ion-dose above 3000 µC/cm^2 allows a higher percentage of the implantation and amorphisation damage to be removed, and leaves less than 0.5% of the gallium remaining in the surface.
We use the ion-milled dishes to form lens moulds; we double-replicate the brittle silicon mould, to create a hard wearing rubber mould. As multiple rubber moulds can be created per silicon mould the process becomes industrially scalable. A thin-film of polymer lenses is then formed from the mould.
We characterise these lenses, demonstrating 1.2-2.5x magnification and resolution of 200nm. We demonstrate their use by imaging two biological samples, one fixed & stained, and one unlabelled in water.
Additionally, using computer simulations alongside the focused ion-beam manufacturing technique, we demonstrate a curved axicon lens structure, that forms long, non-diffracting beams of intense light. We model and experimentally analyse how the lens profile and high-to-low refractive index change forms the beam, and show that increasing the refractive index change decreases the beam width but at a loss of light transmission.
The semiconductor zinc oxide (ZnO) is a promising material for applications in optoelectronics, photochemistry and chemical sensing. Furthermore, ZnO structures can be grown with a large variety of sizes and shapes. Devices with ZnO rods or wires as their core elements can be used in solar cells, gas sensors or biosensors. In this article, an easy approach for the non-aqueous wet chemical synthesis of ZnO structures is presented that employs the solvent trioctylamine (TOA) and the surfactant hexamethylenetetramine (HMTA). Using the thermal decomposition method, rod-shaped structures were grown that are suitable for the fabrication of electrical devices. A detailed study was carried out to investigate the effects of various reaction parameters on the growth process. Both the concentration of the surfactant HMTA and the zinc precursor zincacetylacetonate (Zn(acac)2) were found to show strong effects on the resulting morphology. In addition to structural characterisation using XRD, SEM and TEM, also optical properties of rod-shaped ZnO structures were measured. Rod-shaped structures were obtained for the following conditions: reaction time 4 h, reaction temperature 70 °C, 1 mmol of Zn(acac)2, 4 mmol of HMTA and 25 mL of the solvent TOA. Photoluminescence and photoluminescence excitation spectroscopy of samples grown under these conditions provided information on levels of defect states that could be critical for chemical sensing applications. Two narrow peaks around 254 and 264 nm were found that are well above the band gap of ZnO.
Graphene, a single layer of sp2-bonded carbon atoms, has received significant attention due to its exceptional opto-electronic properties and potentially scalable production processes. However, scalable graphene requires an underlying substrate, which is often a source of strain, doping and carrier scattering, limiting the mobility and quality of graphene. It was shown that by intercalating graphene on SiC by hydrogen, the interfacial layer, associated with n-doping and mobility degradation, is de-coupled from the substrate. The transformations of the H2-intercalation were demonstrated using Raman spectroscopy, while the SiC/interface changes were probed using surface enhanced Raman scattering. The H2-intercalation resulted in carrier type inversion, where the decoupled graphene change from n- to p-type, as well as showing mobility enhancement, up to more than four times, compared to as-grown graphene. Using calibrated Kelvin probe force microscopy, local work function maps were generated, demonstrating the changes in local electronic properties with nanoscale resolution. Furthermore, the layer structure, doping and strain induced by the underlying substrate are compared to CVD grown graphene transferred onto Si/SiO2.
In addition to the substrate effects, the electronic properties of graphene are also significantly affected due to the direct exposure of À electrons to the environment. For the investigation of the environmental effects on graphene (i.e. H2O and NO2), a custom-built environmental transport properties measurement system was designed and developed, allowing magneto-transport measurements to be conducted in highly controlled environments. Using this system and calibrated local work function mapping, it is demonstrated that water withdraws electrons from graphene on SiC and SiO2 substrates, as well as acting as a source of impurity scattering. However, the sensitivity of graphene to water depends highly on the underlying substrate and substrate-induced doping. Moreover, it is shown that epitaxial graphene can successfully be used as the sensing material with detection down to 10 parts-per-billion molecules. Considering the environmental effects on the electronic properties of graphene, the importance of clearly reporting the measurement environmental conditions is high-lighted, whenever a routine characterisation for carrier concentration and mobility is reported.
Metal-organic frameworks (MOFs) have emerged as an exciting class of porous materials that can
be structurally designed by choosing particular components according to desired applications. De-
spite the wide interest in and many potential applications of MOFs, such as in gas storage, catal-
ysis, sensing and drug delivery, electrical semiconductivity and its control is still rare. The use
and fabrication of electronic devices with MOF-based components has not been widely explored,
despite the significant progress of these components made in recent years. Here we report the syn-
thesis and properties of a new highly crystalline, electrochemically active, cobalt and naphthalene
diimide-based MOF that is an efficient electrical semiconductor and has a broad absorption spec-
trum, from 300 nm to 2500 nm. Its semiconductivity was determined by direct voltage bias using
a four-point device, and it features a wavelength dependant photoconductive-photoresistive dual
behaviour, with a very high responsivity of 2.5×105 A W?1.
Li Bo, Godfrey T, Cox David, Li T, Gallop J, Galer Sebastian, Nisbet Andrew, Romans E, Hao L (2018) Investigation of properties of nanobridge Josephson junctions and superconducting tracks fabricated by FIB,Journal of Physics: Conference Series - proceedings of the Advances in Quantum Transport in Low Dimensional Systems conference, 4?5 September 2017, London, UK 964 (Conf.1) 012004 pp. 1-6
Institute of Physics
An important requirement across a range of sensitive detectors is to determine accurately the energy deposited by the impact of a particle in a small volume. The particle may be anything from a visible photon through to an X-ray or massive charged particle. We have been developing nanobridge Josephson junctions based SQUIDs and nanoSQUID devices covering the entire range of particle detection energies from 1eV to MeV. In this paper we discuss some developments in nanobridge Josephson junctions fabrication using focussed ion beam (FIB) and how these developments impact future applications. We focus on tuning of the transition temperature of a superconducting thin-film absorber, with the aim to match the absorber Tc to the working temperature range of the SQUID and also on using a new Xe FIB to improve Josephson junction and superconducting film quality.
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.
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.
Silva S. R. P., Carey JD, Chen G. Y., Cox D. C., Forrest R. D., Poa C. H., Smith R. C., Tang Y. F., Shannon J. M. (2004) Nanoengineering of materials for field emission display technologies,IEE Proceedings in Circuits, Devices and Systems 51 pp. 489-496
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.
The thermal dissolution and decarburization of WC-based powders that occur in various spray processes are a widely studied phenomenon, and mechanisms that describe its development have been proposed. However, the exact formation mechanism of decarburization products such as metallic W is not yet established. A WC-17Co coating is sprayed intentionally at an exceedingly long spray distance to exaggerate the decarburization effects. Progressive xenon plasma ion milling of the examined surface has revealed microstructural features that would have been smeared away by conventional polishing. Serial sectioning provided insights on the three-dimensional structure of the decarburization products. Metallic W has been found to form a shell around small splats that did not deform significantly upon impact, suggesting that its crystallization occurs during the in-flight stage of the particles. W2C crystals are more prominent on WC faces that are in close proximity with splat boundaries indicating an accelerated decarburization in such sites. Porosity can be clearly categorized in imperfect intersplat contact and oxidation-generated gases via its shape.
Quantum computers have been proposed that exploit entangled quantum states between atoms that are isolated from environmental perturbations in a ?semiconductor vacuum? which can be formed by cryogenically cooling an isotopically pure, defect free crystalline layer consisting of Si, or Ge. In a preliminary investigation of an implant and deposition layer exchange technique to produce such ?vacuums?, a layer of aluminium was implanted with 28Si using a conventional implanter. After annealing and cross sectioning, layer exchange was observed to have produced multiple isolated crystals in a cross sectional TEM image. Further deposited Al layers were implanted with Ge using a SIMPLE (Single Ion Multispecies Positioning at Low Energy) implanter over a range of fluences. After anneals at 250/°C and Al removal, crystals of Ge (which also contained Si) were seen at areal densities that increased with implant fluence.
Carbon nanotubes (CNTs) can be used in many different applications. Field emission (FE) measurements were used together with Raman spectroscopy to show a correlation between the microstructure and field emission parameters. However, field emission characterization does not suffer from fluorescence noise present in Raman spectroscopy. In this study, Raman spectroscopy is used to characterize vertically aligned CNT forest samples based on their D/G band intensity ratio (ID/IG), and FE properties such as the threshold electric field, enhancement coefficient, and anode to CNT tip separation (ATS) at the outset of emission have been obtained. A relationship between
ATS at first emission and the enhancement factor, and, subsequently, a relationship between ATS and the ID/IG are shown. Based on the findings, it is shown that a higher enhancement factor (ý3070) results when a lower ID/IG is present (0.45), with initial emissions at larger
distances (ý47 lm). For the samples studied, the morphology of the CNT tips did not play an important role; therefore, the field enhancement factor (b) could be directly related to the carbon nanotube structural properties such as breaks in the lattice or amorphous carbon content. Thus, this work presents FE as a complementary tool to evaluate the quality of CNT samples, with the advantages of alarger probe size and an averaging over the whole nanotube length. Correspondingly, one can find the best field emitter CNT according to its ID/IG.