Vlad Stolojan

Dr Vlad Stolojan


Biography

Biography

Dr Vlad Stolojan is a Senior Lecturer in Nanomaterials Characterisation, working in the Advanced Technology Institute as part of the Nano-Electronics Centre. He is a member of the Institute of Physics and a fellow of the Royal Microscopical Society.

Dr Stolojan is an alumnus of University of East Anglia (BSc Physics -1996) and of the University of Cambridge (PhD Physics -2001 Nanochemistry of grain boundaries in iron"). He first joined the University of Surrey's School of Engineering in 2001 as an expert in electron microscopy and energy-loss spectroscopy, continuing as an RCUK Fellow with the University of Surrey's Electrical and Electronic Engineering department, in Prof Ravi Silva's Nano Electronics Centre.

Dr Stolojan is an author of over 100 peer-reviewed publications and a reviewer for a number of journals (Carbon, Applied Surface Science, Ultramicroscopy, etc.).

Research interests

  • Large area electrospinning of aligned carbon nanotube-polymer composites;
  • Catalytic growth of carbon nanotubes and graphene;
  • Electron Microscopy and associated spectroscopies, particularly Electron Energy-Loss Spectroscopy;
  • Focussed Ion Beam Microscopy, Fabrication and manipulation;
  • Optical microlenses and concentrators;
  • Photovoltaic devices.

PhD Projects are available in:

Teaching

  • EEEM050 - Nanofabrication and Characterisation (Module Co-ordinator): This module covers data processing and analysis, microscopy image processing and analysis, optical spectroscopies, carbon nanotube growth mechanisms, thin film deposition techniques, ion-beam fabrication, implantation and analysis and journal article writing.
  • EEE3037 - Nanoscience and Nanotechnology (with Dr JD Carey): Dr Stolojan's component covers the use of electron and ion microscopy and spectroscopy in nanotechnology.
  • Dr Stolojan is an academic supervisor for Multi-Disciplinary Design Projects, a tutor for FHEQ Levels 4 and 5 EEE students and and contributes to the Engineering Design and Professional Skills.

Departmental duties

  • ATI Health and Safety Academic Representative
  • Academic Exams Team Leader for the Electrical and Electronic Engineering depratment.
  • Academic in charge of the Transmission Electron Microscopes (STEM, TEM) in Materials Structural Studies Unit (MSSU), and the microscopy and analysis suite at the ATI: SEM, ESEM, Dual-Beam FIB, AFM, Scanning Tunneling Microscope (STM) with XPS, UPS and an SEM column.

Affiliations

Institute of Physics (EMAG and The Carbon Group): MInstP.

Fellow of the Royal Microscopical Society.

University roles and responsibilities

  • Senior Lecturer
  • Senior Exams Officer
  • Fire Safety Officer
  • Academic tutor (all years)
  • Academic Supervisor (PhD and EngD)
  • Academic examiner

    My qualifications

    2001
    PhD cantab
    University of Cambridge
    1996
    BSc Hons (1st)
    University of East Anglia

    Affiliations and memberships

    MInstP
    Member of the Institute of Physics (EMAG and Carbon groups)
    Fellow RMS
    Fellow of the Royal Microscopical Society
    Fellow HEA
    Fellow of the Higher Education Academy

    My publications

    Publications

    R Saran, V Stolojan, RJ Curry (2014)Ultrahigh Performance C-60 Nanorod Large Area Flexible Photoconductor Devices via Ultralow Organic and Inorganic Photodoping, In: SCIENTIFIC REPORTS4ARTN 5pp. ?-? NATURE PUBLISHING GROUP
    M Castellino, V Stolojan, A Virga, M Rovere, K Cabiale, MR Galloni, A Tagliaferro (2013)Chemico-physical characterisation and in vivo biocompatibility assessment of DLC-coated coronary stents, In: ANALYTICAL AND BIOANALYTICAL CHEMISTRY405(1)pp. 321-329 SPRINGER HEIDELBERG
    MT Langridge, DC Cox, RP Webb, V Stolojan (2014)The fabrication of aspherical microlenses using focused ion-beam techniques, In: MICRON57pp. 56-66 PERGAMON-ELSEVIER SCIENCE LTD
    P Sharma, V Stolojan, SRP Silva (2011)Raman analysis of oxide cladded silicon core nanowires grown with solid silicon feed stock, In: JOURNAL OF NANOPARTICLE RESEARCH13(7)pp. 2697-2703 SPRINGER
    AJ Smith, RM Gwilliam, V Stolojan, AP Knights, PG Coleman, A Kallis, SH Yeong (2009)Enhancement of phosphorus activation in vacancy engineered thin silicon-on-insulator substrates, In: J APPL PHYS106(10)103514 AMER INST PHYSICS

    The concentration of vacancy-type defects in a silicon-on-insulator substrate consisting of a 110 nm overlayer and a 200 nm buried oxide has been quantified using variable energy positron annihilation spectroscopy following 300 keV Si+ ion implantation to a dose of 1.5 x 10(15) cm(-2) and subsequent, annealing at temperatures ranging from 300 to 700 degrees C. The preferential creation of vacancies (relative to interstitials) in the silicon overlayer leads to a net vacancy-type defect concentration after annealing. Assuming that the defects have a structure close to that. of the divacancy we determine the concentration to range from 1.7 x 10(19) to 5 x 10(18) cm(-3) for annealing temperatures ranging from 300 to 700 degrees C. The measured defect concentration is in excellent agreement with that predicted via Monte Carlo simulation. The impact of this net vacancy population on the diffusion and activation of phosphorus introduced by a 2 keV implantation to a dose of 1 x 10(15) cm(-2) has been observed. For samples that combine both Si+ and P+ implantations, postimplantation phosphorus diffusion is markedly decreased relative to that for P+ implantation only. Further, a fourfold increase in the electrical activation of phosphorus after postimplantation annealing at 750 degrees C is observed when both implantations of Si+ and P+ are performed. We ascribe this affect to the reduction in phosphorus-interstitial clusters by the excess vacancy concentration beyond the amorphous/crystalline interface created by the P+ implantation. (C) 2009 American Institute of Physics. [doi:10.1063/1.3262527]

    P Sharma, JV Anguita, V Stolojan, SJ Henley, SRP Silva (2010)The growth of silica and silica-clad nanowires using a solid-state reaction mechanism on Ti, Ni and SiO2 layers, In: NANOTECHNOLOGY21(29)ARTN 2pp. ?-? IOP PUBLISHING LTD
    AADT Adikaari, JD Carey, V Stolojan, JL Keddie, SRP Silva (2006)Bandgap enhancement of layered nanocrystalline silicon from excimer laser crystallization, In: NANOTECHNOLOGY17(21)pp. 5412-5416 IOP PUBLISHING LTD

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

    NG Shang, YY Tan, V Stolojan, P Papakonstantinou, SRP Silva (2010)High-rate low-temperature growth of vertically aligned carbon nanotubes, In: NANOTECHNOLOGY21(50)ARTN 5pp. ?-? IOP PUBLISHING LTD
    N Peng, C Jeynes, MJ Bailey, D Adikaari, V Stolojan, RP Webb (2009)High concentration Mn ion implantation in Si, In: NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS267(8-9)pp. 1623-1625 ELSEVIER SCIENCE BV
    Nicolas, Van Luc Busatto, Vlad Stolojan, M Shaw, Joseph Keddie, Peter Roth (2018)Reactive Polymorphic Nanoparticles: Preparation via Polymerization-induced Self-assembly and Post-synthesis Thiol–para-Fluoro Core Modification, In: Macromolecular Rapid Communications Wiley-VCH Verlag

    The use of 2,3,4,5,6-pentafluorobenzyl methacrylate (PFBMA) as a core-forming monomer in ethanolic RAFT dispersion polymerization formulations is presented. Poly[poly(ethylene glycol) methyl ether methacrylate] (pPEGMA) macromolecular chain transfer agents were chain extended with PFBMA leading to nanoparticle formation via polymerization-induced self-assembly (PISA). pPEGMA-pPFBMA particles exhibited the full range of morphologies (spheres, worms, and vesicles) including pure and mixed phases. Worm phases formed gels that underwent a thermo-reversible degelation and morphological transition to spheres (or spheres and vesicles) upon heating. Post-synthesis, the pPFBMA cores were modified through thiol–para-fluoro substitution reactions in ethanol using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the base. For monothiols, conversions were 64% (1-octanethiol) and 94% (benzyl mercaptan). Spherical and worm-shaped nano-objects were core cross-linked using 1,8-octanedithiol, which prevented their dissociation in non-selective solvents. For a temperature-responsive worm sample, cross-linking additionally resulted in the loss of the temperature-triggered morphological transition. The use of the reactive monomer PFBMA in PISA formulations presents a simple method to prepare well-defined nano-objects similar to those produced with non-reactive monomers (e.g. benzyl methacrylate) and to retain morphologies independent of solvent and temperature.

    V Stolojan (2008)Scanning Transmission Electron Microscopy: A Tool for Biology and Materials Science, In: Microscopy and Analysis22(4)pp. 15-18 John Wiley & sons

    A dedicated scanning transmission electron microscope is ideally coupled with energy dispersive x-ray and electron energy loss spectroscopies to obtain information about the chemical composition, morphology and electronic structure on the nanoscale. With several signals being available simultaneously with the pass of a sub-nanometresized beam, this instrument can answer questions from a broad range of research areas, in a timely fashion. The user-friendliness of the instrument comes at almost no cost in performance, making it an ideal multi-tool in a teaching environment.

    M Ahmad, JV Anguita, V Stolojan, JD Carey, SR Silva (2013)Efficient coupling of optical energy for rapid catalyzed nanomaterial growth: high-quality carbon nanotube synthesis at low substrate temperatures., In: ACS Appl Mater Interfaces5(9)pp. 3861-3866

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

    NK Mudugamuwa, AADT Adikaari, DMNM Dissanayake, V Stolojan, SRP Silva (2008)Reversible increase of photocurrents in excimer laser-crystallized silicon solar cells, In: SOLAR ENERGY MATERIALS AND SOLAR CELLS92(11)pp. 1378-1381 ELSEVIER SCIENCE BV
    V Stolojan, Y Tison, GY Chen, R Silva (2006)Controlled growth-reversal of catalytic carbon nanotubes under electron-beam irradiation., In: Nano Lett6(9)pp. 1837-1841

    The growth of carbon nanotubes from Ni catalysts is reversed and observed in real time in a transmission electron microscope, at room temperature. The Ni catalyst is found to be Ni3C and remains attached to the nanotube throughout the irradiation sequence, indicating that C most likely diffuses on the surface of the catalyst to form nanotubes. We calculate the energy barrier for saturating the Ni3C (2-13) surface with C to be 0.14 eV, thus providing a low-energy surface for the formation of graphene planes.

    WM Tsang, V Stolojan, AADT Adikaari, SP Wong, SRP Silva (2005)Ion-beam-synthesized Ag-SiO2 nanocomposite layers for electron field emission devices, In: Materials Research Society Symposium Proceedings908pp. 81-86

    Ag-SiO2 nanocomposite layers were synthesised by Ag+ implantation into thermally oxidised SiO2 layers and demonstrated to have excellent field emission (FE) properties. These nanocomposite layers can give an emission current of 1 nA at electric fields less than 20 V/μm, compared to several thousand volts per micrometre of pure metal surfaces. Their fabrication processes are fully compatible with existing integrated circuit technology. By correlating the FE results with other characterisation techniques including atomic force microscopy, Rutherford backscattering spectroscopy and transmission electron microscopy, it is clearly demonstrated that there are two types of field enhancement mechanisms responsible for the excellent FE properties of these cathodes. Firstly, the electrically conductive Ag nano-clusters embedded in the insulating SiO2 matrix give rise to a local electric field enhancement due to an electrical inhomogeneity effect and secondly, the dense surface protrusions provide a geometric local electric field enhancement. The FE properties of these layers are critically dependent on the size and distribution of the Ag clusters, which can be controlled by the Ag dose and modified by the post-implantation pulse annealing with a high power KrF Excimer laser operating at 248 nm. © 2006 Materials Research Society.

    WM Tsang, AADT Adikaari, V Stolojan, B Sealy, SP Wong, SRP Silva (2007)Improving the electron emission properties of ion-beam-synthesized Ag-SiO2 nanocomposites by pulsed laser annealing, In: Technical Digest of the 20th International Vacuum Nanoelectronics Conference, IVNC 07pp. 102-103
    I Yahya, V Stolojan, S Clowes, SM Mustaza, SRP Silva (2010)Carbon nanotube field effect transistor measurements in vacuum, In: IEEE Proceedings of International Conference on Semiconductor Electronicspp. 224-228

    Three terminal measurements on a carbon nanotube field effect transistor (CNTFET) were carried out in high vacuum and the ambient, and its performance compared. The on-off current ratio, ION/IOFF, were 102 and 105 for devices operated in high vacuum and in ambient air, respectively. Here, we show that the conversion of p-type to ambipolar behavior may largely be attributed to the O2 in ambient doping the single walled carbon nanotubes (SWCNTs) in the active channel which consists of bundles of SWCNTs. Switching behaviour of these devices, with respect to constituent types of SWCNTs in the bundles will be discussed.

    V Stolojan (2006)Nanomaterials: Nanotube growth surface-driven, In: Advanced Composites Bulletin(NOV.)pp. 3-?

    Using electron beam irradiation in an electron microscope, researchers at the Advanced Technology Institute, University of Surrey, UK, obtained evidence for the relationship between catalyst and carbon in the growth of carbon nanotubes. By considering the effects of heating and irradiation, the group observed that the carbon atoms at the catalyst surface are easily removed followed by a rapid rearrangement of the nanotube's atoms around the catalyst. Furthermore, they discovered that changes in the nanotube's growth direction are linked to a sudden rotation of the catalyst.

    F Ohashi, GY Chen, V Stolojan, SRP Silva (2008)Influences of hydrogen gas on carbon nanotube growth, In: Materials Research Society Symposium Proceedings1081pp. 87-93

    For practical deployment of carbon nanotubes, an understanding of their growth mechanism is required in order to obtain better control over their crystallinity, chirality and other structural properties. In this study, we focus on the influences of gas species on carbon nanotube synthesis using thermal chemical vapour deposition. The influence of methane, hydrogen, and helium gases was investigated from the viewpoint of gas chemistry in relation to the nanotube structural change, by varying the growth pressure, the gas-flow ratio and the growth temperature. Simple changes in the hydrogen gas concentration during different growth stages have been found to induce surprising changes to the nanotube formation. The structure of the tubular carbon growth changed from amorphous to graphitic as the growth temperature and the concentration of hydrogen in the initial periods of growth decreases. The excess hydrogen tends to give rise to poor crystalline carbon nanofibres but has the effect of increasing the yields. Hydrogen gas is typically used in reducing metal catalyst particles during the pre-treatment and the carbon nanotube growth periods. We show that while hydrogen species can improve yield, it can also result in the degradation of the nanotube's crystallinity. The use of hydrogen in the growth process is one of the key parameters for enhanced control of carbon nanotube/nanofibre growth and their resulting crystallinity.

    LC Chong, V Stolojan, G Wagner, SRP Silva, RJ Curry (2008)One-pot rapid low-cost synthesis of Pd-fullerite catalysts, In: JOURNAL OF MATERIALS CHEMISTRY18(40)pp. 4808-4813 ROYAL SOC CHEMISTRY
    M Constantinou, V Stolojan, KP Rajeev, S Hinder, B Fisher, TD Bogart, BA Korgel, M Shkunov (2015)Interface Passivation and Trap Reduction via a Solution-Based Method for Near-Zero Hysteresis Nanowire Field-Effect Transistors, In: ACS APPLIED MATERIALS & INTERFACES7(40)pp. 22115-22120 AMER CHEMICAL SOC

    In this letter, we demonstrate a solution-based method for a one-step deposition and surface passivation of the as-grown silicon nanowires (Si NWs). Using N,N-dimethylformamide (DMF) as a mild oxidizing agent, the NWs' surface traps density was reduced by over 2 orders of magnitude from 1×10(13) cm(-2) in pristine NWs to 3.7×10(10) cm(-2) in DMF-treated NWs, leading to a dramatic hysteresis reduction in NW field-effect transistors (FETs) from up to 32 V to a near-zero hysteresis. The change of the polyphenylsilane NW shell stoichiometric composition was confirmed by X-ray photoelectron spectroscopy analysis showing a 35% increase in fully oxidized Si4+ species for DMF-treated NWs compared to dry NW powder. Additionally, a shell oxidation effect induced by DMF resulted is a more stable NW FET performance with steady transistor currents and only 1.5 V hysteresis after 1000 h of air exposure

    Simon King, Evandro Castaldelli, Liam McCaffterty, S Ravi P Silva, Vlad Stolojan (2018)A Micro-Centrifugal Technique for Improved Assessment and Optimization of Nanomaterial Dispersions: The Case for Carbon Nanotubes, In: ACS Applied Nano Materials1(11)pp. 6217-6225 American Chemical Society

    Large-scale incorporation of nanomaterials into manufactured materials can only take place if they are suitably dispersed and mobile within the constituent components, typically within a solution/ink formulation so that the additive process can commence. Natural hydrophobicity of many nanomaterials must be overcome for their successful incorporation into any solution-based manufacturing process. To date, this has been typically achieved using polymers or surfactants, rather than chemical functionalization, to preserve the remarkable properties of the nanomaterials. Quantifying surfactant or dispersion technique efficacy has been challenging. Here we introduce a new methodology to quantify dispersions applicable to high-weight fraction suspensions of most nanomaterials. It’s based on centrifuging and weighing residue of undispersed material. This enables the determination of the efficacy of surfactants to disperse nanomaterials (e.g. ultrasonication power and duration) and leads to increased nanomaterial solution loading. To demonstrate this technique, we assessed carbon nanotube dispersions using popular surfactants: Benzalkonium chloride (ADBAC), Brij®52, Brij®58, Pluronic®F127, sodium dodecyl sulfate (SDS), sodium dodecylbenzenesulfonate (SDBS), Triton™ X-100, Triton™X-405 and Tween®80, evaluating the dispersion outcome when varying sonicator power and horn depth, as well as imaging sono-intensity within the solution with luminol. The methodology is shown to be applicable for high-weight fraction nanomaterial suspensions, enabling greater deployment.

    GD Dabera, KD Jayawardena, MR Prabhath, I Yahya, YY Tan, NA Nismy, H Shiozawa, M Sauer, G Ruiz-Soria, P Ayala, V Stolojan, AA Adikaari, PD Jarowski, T Pichler, SR Silva (2013)Hybrid carbon nanotube networks as efficient hole extraction layers for organic photovoltaics., In: ACS Nano7(1)pp. 556-565

    Transparent, highly percolated networks of regioregular poly(3-hexylthiophene) (rr-P3HT)-wrapped semiconducting single-walled carbon nanotubes (s-SWNTs) are deposited, and the charge transfer processes of these nanohybrids are studied using spectroscopic and electrical measurements. The data disclose hole doping of s-SWNTs by the polymer, challenging the prevalent electron-doping hypothesis. Through controlled fabrication, high- to low-density nanohybrid networks are achieved, with low-density hybrid carbon nanotube networks tested as hole transport layers (HTLs) for bulk heterojunction (BHJ) organic photovoltaics (OPV). OPVs incorporating these rr-P3HT/s-SWNT networks as the HTL demonstrate the best large area (70 mm(2)) carbon nanotube incorporated organic solar cells to date with a power conversion efficiency of 7.6%. This signifies the strong capability of nanohybrids as an efficient hole extraction layer, and we believe that dense nanohybrid networks have the potential to replace expensive and material scarce inorganic transparent electrodes in large area electronics toward the realization of low-cost flexible electronics.

    CE Giusca, Y Tison, V Stolojan, E Borowiak-Palen, SRP Silva (2007)Inner-tube chirality determination for double-walled carbon nanotubes by scanning tunneling microscopy, In: NANO LETTERS7(5)pp. 1232-1239 AMER CHEMICAL SOC
    TR Pozegic, JV Anguita, I Hamerton, KDGI Jayawardena, J-S Chen, V Stolojan, P Ballocchi, R Walsh, SRP Silva (2016)Multi-Functional Carbon Fibre Composites using Carbon Nanotubes as an Alternative to Polymer Sizing, In: Scientific Reports637334

    Carbon fibre reinforced polymers (CFRP) were introduced to the aerospace, automobile and civil engineering industries for their high strength and low weight. A key feature of CFRP is the polymer sizing - a coating applied to the surface of the carbon fibres to assist handling, improve the interfacial adhesion between fibre and polymer matrix and allow this matrix to wet-out the carbon fibres. In this paper, we introduce an alternative material to the polymer sizing, namely carbon nanotubes (CNTs) on the carbon fibres, which in addition imparts electrical and thermal functionality. High quality CNTs are grown at a high density as a result of a 35 nm aluminium interlayer which has previously been shown to minimise diffusion of the catalyst in the carbon fibre substrate. A CNT modified-CFRP show 300%, 450% and 230% improvements in the electrical conductivity on the ‘surface’, ‘through-thickness’ and ‘volume’ directions, respectively. Furthermore, through-thickness thermal conductivity calculations reveal a 107% increase. These improvements suggest the potential of a direct replacement for lightning strike solutions and to enhance the efficiency of current de-icing solutions employed in the aerospace industry.

    V Nicolosi, PD Nellist, S Sanvito, EC Cosgriff, S Krishnamurthy, WJ Blau, MLH Green, D Vengust, D Dvorsek, D Mihailovic, G Compagnini, J Sloan, V Stolojan, JD Carey, SJ Pennycook, JN Coleman (2007)Observation of van der Waals driven self-assembly of MoSI nanowires into a low-symmetry structure using aberration-corrected electron microscopy, In: ADVANCED MATERIALS19(4)pp. 543-+ WILEY-V C H VERLAG GMBH

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

    Radu A. Sporea, Luke J. Wheeler, Vlad Stolojan, S. Ravi P. Silva (2018)Towards manufacturing high uniformity polysilicon circuits through TFT contact barrier engineering, In: Scientific Reports Nature Research

    Abstract The predicted 50 billion devices connected to the Internet of Things by 2020 has renewed interest in polysilicon technology for high performance new sensing and control circuits, in addition to traditional display usage. Yet, the polycrystalline nature of the material presents significant challenges when used in transistors with strongly scaled channel lengths due to non-uniformity in device performance. For these new applications to materialize as viable products, uniform electrical characteristics on large areas will be essential. Here, we report on the effect of deliberately engineered potential barrier at the source of polysilicon thin-film transistors, yielding highly-uniform on-current (

    DC Cox, RD Forrest, PR Smith, V Stolojan, SRP Silva (2005)Study of the current stressing in nanomanipulated three-dimensional carbon nanotube structures, In: APPLIED PHYSICS LETTERS87(3)ARTN 0pp. ?-? AMER INST PHYSICS
    DA Zeze, JD Carey, V Stolojan, BL Weiss, SRP Silva (2006)Damage effects in Pyrex by CF4 reactive ion etching in dual RF-microwave plasmas, In: MICRO & NANO LETTERS1(2)pp. 103-107 INST ENGINEERING TECHNOLOGY-IET

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

    T Katsuno, S Nitta, H Habuchi, V Stolojan, SRP Silva (2004)Highly photoconductive amorphous carbon nitride films prepared by cyclic nitrogen radical sputtering, In: APPLIED PHYSICS LETTERS85(14)pp. 2803-2805 AMER INST PHYSICS
    Y Jin, RJ Curry, J Sloan, RA Hatton, LC Chong, N Blanchard, V Stolojan, HW Kroto, SRP Silva (2006)Structural and optoelectronic properties of C-60 rods obtained via a rapid synthesis route, In: JOURNAL OF MATERIALS CHEMISTRY16(37)pp. 3715-3720 ROYAL SOC CHEMISTRY
    CE Giusca, V Stolojan, J Sloan, H Shiozawa, SRP Silva (2010)GeTe-filled carbon nanotubes for data storage applications, In: MRS Proceedings1251pp. 19-24

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

    HM Thirimanne, KDGI Jayawardena, JV Anguita, V Stolojan, CA Mills, SRP Silva (2016)Charge Funneling through Metal Electrode Structuring for High-Efficiency Gains in Polymer Solar Cells, In: ADVANCED ELECTRONIC MATERIALS2(9)ARTN 16000 WILEY-BLACKWELL
    GY Chen, B Jensen, V Stolojan, SRP Silva (2011)Growth of carbon nanotubes at temperatures compatible with integrated circuit technologies, In: Carbon49(1)pp. 280-285 Elsevier
    M Palumbo, SJ Henley, T Lutz, V Stolojan, D Cox, SRP Silva (2008)Engineering the shape of zinc oxide crystals via sonochemical or hydrothermal solution-based methods, In: Materials Research Society Proceedings1087pp. 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.

    M Constantinou, GP Rigas, FA Castro, V Stolojan, KF Hoettges, M Hughes, E Adkins, BA Korgel, M Shkunov (2016)Simultaneous Tunable Selection and Self-Assembly of Si Nanowires from Heterogeneous Feedstock, In: ACS Nano10(4)pp. 4384-4394 American Chemical Society

    Semiconducting nanowires (NWs) are becoming essential nano-building blocks for advanced devices from sensors to energy harvesters, however their full technology penetration requires large scale materials synthesis together with efficient NW assembly methods. We demonstrate a scalable one-step solution process for the direct selection, collection and ordered assembly of silicon NWs with desired electrical properties from a poly-disperse collection of NWs obtained from a Supercritical Fluid-Liquid-Solid growth process. Dielectrophoresis (DEP) combined with impedance spectroscopy provides a selection mechanism at high signal frequencies (>500 kHz) to isolate NWs with the highest conductivity and lowest defect density. The technique allows simultaneous control of five key parameters in NW assembly: selection of electrical properties, control of NW length, placement in pre-defined electrode areas, highly preferential orientation along the device channel and control of NWs deposition density from few to hundreds per device. Direct correlation between DEP signal frequency and deposited NWs conductivity is directly confirmed by field-effect transistor and conducting-AFM data. Fabricated NW transistor devices demonstrate excellent performance with up to 1.6 mA current, 106-107 on/off ratio and hole-mobility of 50 cm2 V-1 s-1.

    V Stolojan, W Tsang, SRP Silva, AG Cullis, PA Midgley (2008)The Dielectric Properties of Co-Implanted SiO(2) Investigated Using Spatially-Resolved EELS, In: MICROSCOPY OF SEMICONDUCTING MATERIALS 2007120pp. 341-344
    Elizabeth J. Legge, Muhammad Ahmad, Christopher T. G. Smith, Barry Brennan, Christopher A. Mills, Vlad Stolojan, Andrew J. Pollard, S. Ravi P. Silva (2018)Physicochemical characterisation of reduced graphene oxide for conductive thin films, In: RSC Advances8(65)pp. 37540-37549 Royal Society of Chemistry

    Graphene is a desirable material for next generation technology. However, producing high yields of single-layer flakes with industrially applicable methods is currently limited. We introduce a combined process for the reduction of graphene oxide (GO) via vitamin C (ascorbic acid) and thermal annealing at temperatures of

    WM Tsang, V Stolojan, BJ Sealy, SP Wong, SRP Silva (2007)Electron field emission properties of Co quantum dots in SiO2 matrix synthesised by ion implantation, In: ULTRAMICROSCOPY107(9)pp. 819-824

    In this work, Co ions were implanted into thermally oxidised SiO2 layers on silicon substrates. The implantation energy was 50 keV and the doses were 1, 3, 5 and 7 x 10(16) Co+/cm2. The field emission (FE) properties of these layers were studied and correlated with results from atomic force microscopy and transmission electron microscopy measurements. Other than that for the lowest dose sample, crystallised Co nanoclusters, with sizes ranging from 1.8 to 5.7 nm, are observed in these Co-implanted layers. The higher dose samples exhibit excellent FE properties and give an emission current of 1 nA at electric fields as low as 5 V/microm, for a dose of 5 x 10(16) Co+/cm2, compared with 120 V/microm for the lowest dose samples. We attribute the excellent FE properties of these layers to the formation of Co nanoclusters, with the electrical inhomogeneity giving rise to local field enhancement. Finally, repeatable staircase-like current-field (I-F) characteristics are observed in FE measurements of these higher dose samples as compared to conventional Fowler-Nordheim-type I-F characteristics in the lower dose sample. We believe this data may be a result of Coulomb blockade effects arising from the isolated low-capacitance metal quantum dots formed by controlled ion implantation.

    WM Tsang, V Stolojan, S Wong, JKN Linder, BJ Sealy, SRR Silva (2007)The structural and electron field emission properties of ion-beam-synthesised metallic-dielectric nanocomposites, In: REVIEWS ON ADVANCED MATERIALS SCIENCE15(3)pp. 179-184
    Simon King, L McCafferty, Vlad Stolojan, Ravi Silva (2015)Highly aligned arrays of super resilient carbon nanotubes by steam purification, In: Carbon84(1)pp. 130-137

    Steam treatment has been applied to our prefabricated highly aligned areas of electrospun carbon nanotube composite nano-fibres, leading to controlled and targeted removal of polymeric and amorphous carbon materials, resulting in areas of highly aligned, highly crystalline, pure nanotubes. Raman analysis shows how the ID to IG intensity ratio was reduced to 0.03, and the radial breathing mode peak intensity, used for nanotube diameter calculation, changes. Therefore, suggesting that some carbon nanotubes are more resistant to steam assisted oxidation, meaning that specific carbon nanotube diameters are preferentially oxidised. The remaining carbon nanotubes have displayed a significant improvement in both quality, with respect to defect density, and in crystallinity, resulting in an increased resistance to oxidation. These steam treated super resilient carbon nanotubes are shown to withstand temperatures of above 900 °C under ambient conditions. Applying this purification method to electrospun nano-fibres leads the way for the next generation of composite materials which can be used in high temperature extreme environments.

    MJ Whiting, V Stolojan, A Rattana, JF Watts (2003)Direct observation and characterisation of the oxide nanostructured interface resulting from organosilane pre-treatment of aluminium, In: POLYMER/METAL INTERFACES AND DEFECT MEDIATED PHENOMENA IN ORDERED POLYMERS734pp. 27-32
    GY Chen, B Jensen, V Stolojan, SRP Silva (2010)Growth of carbon nanotubes at CMOS compatible temperatures, In: Carbon
    Y Miyajima, Y Tison, CE Giusca, V Stolojan, H Watanabe, H Habuchi, SJ Henley, JM Shannon, SRP Silva (2011)Probing the band structure of hydrogen-free amorphous carbon and the effect of nitrogen incorporation, In: CARBON49(15)pp. 5229-5238
    F Ohashi, GY Chen, V Stolojan, SRP Silva (2008)The role of the gas species on the formation of carbon nanotubes during thermal chemical vapour deposition, In: NANOTECHNOLOGY19(44)ARTN 4pp. ?-? IOP PUBLISHING LTD
    S Gravani, K Polychronopoulou, V Stolojan, Q Cui, PN Gibson, SJ Hinder, Z Gu, CC Doumanidis, MA Baker, C Rebholz (2010)Growth and characterization of ceria thin films and Ce-doped gamma-Al2O3 nanowires using sol-gel techniques, In: NANOTECHNOLOGY21(46)465606 IOP PUBLISHING LTD

    γ-Al2O3 is a well known catalyst support. The addition of Ce to γ-Al2O3 is known to beneficially retard the phase transformation of γ-Al2O3 to α-Al2O3 and stabilize the γ-pore structure. In this work, Ce-doped γ-Al2O3 nanowires have been prepared by a novel method employing an anodic aluminium oxide (AAO) template in a 0.01 M cerium nitrate solution, assisted by urea hydrolysis. Calcination at 500 °C for 6 h resulted in the crystallization of the Ce-doped AlOOH gel to form Ce-doped γ-Al2O3 nanowires. Ce3 + ions within the nanowires were present at a concentration of < 1 at.%. On the template surface, a nanocrystalline CeO2 thin film was deposited with a cubic fluorite structure and a crystallite size of 6–7 nm. Characterization of the nanowires and thin films was performed using scanning electron microscopy, transmission electron microscopy, electron energy loss spectroscopy, x-ray photoelectron spectroscopy and x-ray diffraction. The nanowire formation mechanism and urea hydrolysis kinetics are discussed in terms of the pH evolution during the reaction. The Ce-doped γ-Al2O3 nanowires are likely to find useful applications in catalysis and this novel method can be exploited further for doping alumina nanowires with other rare earth elements.

    GY Chen, V Stolojan, SRP Silva, H Herman, S Haq (2005)Carbon spheres generated in 'dusty plasmas', In: CARBON43(4)pp. 704-708 PERGAMON-ELSEVIER SCIENCE LTD
    KK Devarepally, David Cox, AT Fry, Vlad Stolojan, Richard Curry, M Munz (2011)Synthesis of linear ZnO structures by a thermal decomposition method and their characterisation, In: Journal of Materials Science47(4)pp. 1893-1901 Springer

    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.

    V Stolojan, P Moreau, MJ Goringe, SRP Silva (2006)Subnanometer-resolved measurement of the tunneling effective mass using bulk plasmons, In: APPLIED PHYSICS LETTERS88(12)ARTN 1pp. ?-? AMER INST PHYSICS

    Superlattices are periodic structures where the constituents alternate between low- and high-bandgap materials; the resulting quantum confinement tailors the resulting device properties and increases their operating speed. Amorphous carbon is an excellent candidate for both the well and barrier layers of the superlattices, leading to a fast and reliable device manufacturing process. We show theoretically and experimentally that, using low energy-loss spatially resolved spectroscopy, we can characterize the component layers of a superlattice. We measure quantum confinement of the electron wave function in the superlattice's wells and calculate the effective tunneling mass for amorphous carbon superlattices as m(*)=0.067m(e). This effective mass makes diamondlike carbon films as feasible candidate for electronic devices.

    RD Boyd, TJ Young, V Stolojan (2012)Characterisation of gold nanoparticles and rods using high angle annular dark field imaging, In: JOURNAL OF NANOPARTICLE RESEARCH14(4)ARTN 7pp. ?-? SPRINGER
    GY Chen, V Stolojan, SRP Silva (2010)Top-Down Heating for Low Substrate Temperature Synthesis of Carbon Nanotubes, In: JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY10(6)pp. 3952-3958 AMER SCIENTIFIC PUBLISHERS
    Simon King, Vlad Stolojan, Ravi Silva (2017)Large area uniform electrospun polymer nanofibres by balancing of the electrostatic field, In: Reactive and Functional Polymers129pp. 89-94 Elsevier

    As a technique, electrospinning has been increasingly utilised for polymer nanofibre production, which has a growing list of advanced applications to which they are being applied. However, commercially scaling the process is challenging, especially when the uniformity of the nanofibres across the bulk of the material is important for the required application. At present, most commercially-scalable systems tend to rely on a drum or cylindrical-style electrode, where a multitude of electrospinning jets are formed with no specific controlled distribution or uniformity over its surface. These electrospinning systems also have the drawback of possessing a varying electrostatic field across the length of the electrode, resulting in a range of spinning conditions which result in an inconsistency in the produced nanofibres. Due to the high centrifugal stresses exerted on the polymer during electrospinning, controlling the electrostatic field is crucial for consistent nanofibre production, which forms the basis for applications such as cellular scaffolds and smart materials. In the work reported here, we utilise computational simulation to explore a range of electrode designs to achieve a large area electrospinning system with a balanced electrostatic field across its entire active surface. We demonstrate the output by producing a high-throughput of nanofibres with comparable properties to that of a traditional single spinneret system, but at a processing rate two orders of magnitude faster.

    C Opoku, KF Hoettges, MP Hughes, V Stolojan, SRP Silva, M Shkunov (2013)Solution processable multi-channel ZnO nanowire field-effect transistors with organic gate dielectric, In: Nanotechnology24(40)pp. 405203-4(7pp)

    The present work focuses on nanowire (NW) applications as semiconducting elements in solution processable field-effect transistors (FETs) targeting large-area low-cost electronics. We address one of the main challenges related to NW deposition and alignment by using dielectrophoresis (DEP) to select multiple ZnO nanowires with the correct length, and to attract, orientate and position them in predefined substrate locations. High-performance top-gate ZnO NW FETs are demonstrated on glass substrates with organic gate dielectric layers and surround source-drain contacts. Such devices are hybrids, in which inorganic multiple single-crystal ZnO NWs and organic gate dielectric are synergic in a single system. Current-voltage (I-V) measurements of a representative hybrid device demonstrate excellent device performance with high on/off ratio of 10^7, steep subthreshold swing (s-s) of 400 mV/dec and high electron mobility of 35 cm2 V-1 s-1 in N2 ambient. Stable device operation is demonstrated after 3 months of air exposure, where similar device parameters are extracted including on/off ratio of 4x10^6, s-s 500 mV/dec and field-effect mobility of 28 cm2 V-1 s-1. These results demonstrate that DEP can be used to assemble multiples of NWs from solvent formulations to enable low-temperature hybrid transistor fabrication for large-area inexpensive electronics.

    Y Miyajima, SJ Henley, G Adamopoulos, V Stolojan, E Garcia-Caurel, B Drevillon, JM Shannon, SRP Silva (2009)Pulsed laser deposited tetrahedral amorphous carbon with high sp(3) fractions and low optical bandgaps, In: JOURNAL OF APPLIED PHYSICS105(7)ARTN 0pp. ?-? AMER INST PHYSICS
    E Mendoza, SJ Henley, CHP Poa, V Stolojan, GY Chen, CE Giusca, JD Carey, SRP Silva (2005)Dendrimer assisted catalytic growth of mats of multiwall carbon nanofibers, In: CARBON43(10)pp. 2229-2231 PERGAMON-ELSEVIER SCIENCE LTD
    M Palumbo, SJ Henley, T Lutz, V Stolojan, SRP Silva (2008)A fast sonochemical approach for the synthesis of solution processable ZnO rods, In: JOURNAL OF APPLIED PHYSICS104(7)ARTN 0pp. ?-? AMER INST PHYSICS
    R Duncan, V Stolojan, C Lekakou (2007)Manufacture of carbon multi-walled nanotubes by the arc discharge technique, In: World Congress on Engineering 2007, Vols 1 and 2pp. 1391-1395
    E Borowiak-Palen, MH Ruemmeli, E Mendoza, SJ Henley, DC Cox, CHP Poa, V Stolojan, T Gemming, T Pichler, SRP Silva (2005)Silver intercalated carbon nanotubes, In: Electronic Properties of Novel Nanostructures786pp. 236-239 AMER INST PHYSICS
    N Shang, GY Chen, YY Tan, V Stolojan, P Papakonstantinou, SRP Silva (2010)Direct catalytic growth of high-density carbon nanotubes on nanoclusters at low temperatures, In: Proceedings of 8th International Vacuum Electron Sources Conference and Nanocarbonpp. 389-389

    Carbon nanotubes (CNTs) have received extensive attention due to their one-dimensional structure and ability to demonstrate many novel physical and chemical phenomena in the quantum scale. However, the application of CNTs in electronics is hindered due to their higher growth temperatures which are usually in excess of 500 °C, which is not compatible with current semiconductor technology in industry. Low temperature growth is necessary for integrating CNTs into standard semiconductor devices such as CMOS and large-scale integrated circuits. To date, various techniques have been utilised to lower the CNT growth temperature by: 1. using various carbon sources with lower dissociation temperature; 2. exploring metal catalyst films of the low melting point or metal nanoparticles as catalysts; and, 3. introducing a plasma during deposition to increase the dissociation and ionization of feed gases. In this study, we report the low temperature growth of vertically aligned high-density CNTs by a DC plasma chemical vapour deposition method, using Ni nanoclusters as catalysts. The Ni nanoclusters are free from a high-temperature formation process compared to the film based catalysts and directly demonstrate catalytic growth of CNTs at substrate temperatures as low as 390 °C. The density of as-grown CNTs is up to 10 /cm , as shown in Figure 1. Transmission electron microscopy studies show the CNTs are made of crystalline graphene shells and have a uniform diameter distribution. The field electron emission properties of the samples are investigated.

    K Prabha Rajeev, C Opoku, V Stolojan, M Constantinou, M Shkunov (2017)Effect of Nanowire-dielectric Interface on the Hysteresis of Solution Processed Silicon Nanowire FETs, In: Nanoscience and Nanoengineering5(2)pp. 17-24 Horizon Research Publishing

    Silicon nanowires (Si NW) are ideal candidates for low-cost solution processed field effect transistors (FETs) due to the ability of nanowires to be dispersed in solvents, and demonstrated high charge carrier mobility. The interface between the nanowire and the dielectric plays a crucial role in the FET characteristics, and can be responsible for unwanted effects such as current hysteresis during device operation. Thus, optimal nanowire- dielectric interface is required for low-hysteresis FET performance. Here we show that NW FET hysteresis mostly depends on the nature of the dielectric material by directly comparing device characteristics of dual gate Si NW FETs with bottom SiO2 gate dielectric and top hydrophobic fluoropolymer gate dielectric. As the transistor semiconducting nanowire channel is identical in both tops and bottom operational regimes, the performance differences originate from the nature of the nanowire-dielectric interface. Thus, very high 30 volt hysteresis is observed for forward and reverse gate bias scans with SiO2 interface; however, hysteresis is significantly reduced to 6 volt for the fluoropolymer dielectric interface. The differences in hysteresis are ascribed to the polar OH- groups present at SiO2/Si nanowire interface, and mostly absent at fluoropolymer/Si nanowire interface. We further demonstrate that high density of charge traps for bottom gate SiO2 interface (1× 1013cm-2) is reduced by over an order of magnitude for top-fluoropolymer gate interface (7.5 × 1011 cm-2), therefore highlighting the advantage of hydrophobic polymer gate dielectrics for nanowire field-effect transistor applications.

    GY Chen, PCH Poa, SJ Henley, V Stolojan, SRP Silva (2005)Carbon nanotubes and nanostructures grown at low temperatures below 400°C, In: Materials Research Society Symposium Proceedings901pp. 511-522

    In this paper, we report clear evidence for the growth of carbon nanotubes and nanostructures at low substrate temperatures, using direct-current plasma-enhanced chemical vapour deposition. The catalyst particles are mounted on a titanium layer which acts as a thermal barrier, and allows for a larger temperature gradient between the Ni catalyst surface and the substrate. A simple thermodynamic simulation shows that the temperature differential between the substrate growth surface and the growth electrode is determined by the thickness of the titanium layer. This facilitates the growth of nanotubes, as opposed to nanofibres with herring-bone or amorphous structures. The growth properties are discussed as a function of the bias voltage and hydrocarbon concentration. The heating during growth provided solely by the plasma is below 400°C and is dependent on the process conditions and the electrode configuration in the growth chamber. These conditions need to be taken into account when comparing processes across different growth methods and instruments. The novel approach based on the use of a thermal barrier ensures the synthesis of carbon nanotubes at room temperature substrate conditions, which can be attained with a suitable cooling scheme. © 2006 Materials Research Society.

    Y Miyajima, G Adamopoulos, SJ Henley, V Stolojan, Y Tison, E Garcia-Caurel, B Drevillon, JM Shannon, SRP Silva (2008)Electronic state modification in laser deposited amorphous carbon films by the inclusion of nitrogen, In: JOURNAL OF APPLIED PHYSICS104(6)ARTN 0pp. ?-? AMER INST PHYSICS

    In this study, we investigate the effect of the inclusion of nitrogen in amorphous carbon thin films deposited by pulsed laser deposition, which results in stress induced modifications to the band structure and the concomitant changes to the electronic transport properties. The microstructural changes due to nitrogen incorporation were examined using electron energy-loss spectroscopy and Raman scattering. The band structure was investigated using spectroscopic ellipsometry data in the range of. 1.5-5 eV, which was fitted to the Tauc Lorentz model parametrization and optical transmittance measurements. The dielectric constant evaluated using optical techniques was compared to that obtained with electrical measurements, assuming a Poole-Frenkel type conduction process based on the best fits to data. The electrical conduction mechanism is discussed for both low and high electric fields, in the context of the shape of the band density of states. By relating a wide range of measurement techniques, a detailed relationship between the microstructure, and the optical and the electrical structures of a-CNx films is obtained. From these measurements, it was found that, primarily, the change in density of the film, with increasing nitrogen pressure, affects the band structure of the amorphous carbon nitride. This is due to the fact that the density affects the stress in the film, which also impacts the localized states in the band gap. These results are supported by density of states measurements using scanning tunneling spectroscopy.

    C Opoku, R Sporea, V Stolojan, R Silva, M Shkunov (2015)Si Nanowire - Array Source Gated Transistors

    Solution processed field-effect transistors based on single crystalline silicon nanowires (Si NWs) with metal Schottky contacts are demonstrated. The semiconducting layer was deposited from a nanowire ink formulation at room temperature. The devices with 230nm thick SiO2 gate insulating layers show excellent output current-voltage characteristics with early saturation voltages under 2 volts, constant saturation current and exceptionally low dependence of saturation voltage with the gate field. Operational principles of these devices are markedly different from traditional ohmic-contact field-effect transistors (FETs), and are explained using the source-gated transistor (SGT) concept in which the semiconductor under the reverse biased Schottky source barrier is depleted leading to low voltage pinch-off and saturation of drain current. Device parameters including activation energy are extracted at different temperatures and gate voltages to estimate the Schottky barrier height for different electrode materials to establish transistor performance - barrier height relationships. Numerical simulations are performed using 2D thin-film approximation of the device structures at various Schottky barrier heights. Without any adjustable parameters and only assuming low p-doping of the transistor channel, the modelled data show exceptionally good correlation with the measured data. From both experimental and simulation results, it is concluded that source-barrier controlled nanowire transistors have excellent potential advantages compared with a standard FET including mitigation of short-channel effects, insensitivity in device operating currents to device channel length variation, higher on/off ratios, higher gain, lower power consumption and higher operational speed for solution processable and printable nanowire electronics.

    NLV Carreno, MT Escote, A Valentini, L McCafferty, V Stolojan, M Beliatis, Chris Mills, R Rhodes, Christopher Smith, S Silva (2015)Adsorbent 2D and 3D carbon matrices with protected magnetic iron nanoparticles, In: NANOSCALE7(41)pp. 17441-17449 ROYAL SOC CHEMISTRY
    Alfred Adams, Igor Marko, J Mukherjee, Vlad Stolojan, Stephen Sweeney, A Gocalinska, E Pelucchi, K Thomas, B Corbett (2015)Semiconductor Quantum Well Lasers With a Temperature-Insensitive Threshold Current, In: IEEE Journal of Selected Topics in Quantum Electronics21(6)150080pp. ?-? IEEE

    This paper proposes and demonstrates a new multiquantum well (MQW) laser structure with a temperature-insensitive threshold current and output power. Normally, the mechanisms that cause the threshold current (Ith) of semiconductor lasers to increase with increasing temperature T (thermal broadening of the gain spectrum, thermally activated carrier escape, Auger recombination, and intervalence band absorption) act together to cause Ith to increase as T increases. However, in the design presented here, carriers thermally released from some of the QWs are fed to the other QWs so that these mechanisms compensate rather than augment one another. The idea is in principle applicable to a range of materials systems, structures, and operating wavelengths. We have demonstrated the effect for the first time in 1.5 μm GaInAsP/InP Fabry-Perot cavity edge-emitting lasers. The results showed that it is possible to keep the threshold current constant over a temperature range of about 100 K and that the absolute temperature over which the plateau occurred could be adjusted easily by redesigning the quantum wells and the barriers between them. TEM studies of the structures combined with measurements of the electroluminescent intensities from the wells are presented and explain well the observed effects.

    Mehmet O. Tas, Mark A. Baker, Vishal Musaramthota, Hasan Uppal, Mateus G. Masteghin, Jedidiah Bentz, Keir Boxshall, Vlad Stolojan (2019)Carbon nanotube micro-contactors on ohmic substrates for on-chip microelectromechanical probing applications at wafer level, In: Carbon150pp. 117-127 Elsevier

    CNTs can have the ability to act as compliant small-scale springs or as shock resistance micro-contactors. This work investigates the performance of vertically-aligned CNTs (VA-CNTs) as micro-contactors in electromechanical testing applications for testing at wafer-level chip-scale-packaging (WLCSP) and wafer-level-packaging (WLP). Fabricated on ohmic substrates, 500-μm-tall CNT-metal composite contact structures are electromechanically characterized. The probe design and architecture are scalable, allowing for the assembly of thousands of probes in short manufacturing times, with easy pitch control. We discuss the effects of the metallization morphology and thickness on the compliance and electromechanical response of the metal-CNT composite contacts. Pd-metallized CNT contactors show up to 25 μm of compliance, with contact resistance as low as 460 mΩ (3.6 kΩ/μm) and network resistivity of 1.8 × 10−5 Ω cm, after 2500 touchdowns, with 50 μm of over-travel; they form reproducible and repeatable contacts, with less than 5% contact resistance degradation. Failure mechanisms are studied in-situ and after cyclic testing and show that, for top-cap-and-side metallized contacts, the CNT-metal shell provides stiffness to the probe structure in the elastic region, whilst reducing the contact resistance. The stable low resistance achieved, the high repeatability and endurance of the manufactured probes make CNT micro-contacts a viable candidate for WLP and WLCSP testing.

    WM Tsang, SJ Henley, V Stolojan, SRP Silva (2006)Negative differential conductance observed in electron field emission from band gap modulated amorphous-carbon nanolayers, In: APPLIED PHYSICS LETTERS89(19)ARTN 1pp. ?-? AMER INST PHYSICS
    J Bertho, V Stolojan, M-L Abel, JF Watts (2010)The effect of silane incorporation on a metal adhesive interface: A study by electron energy loss spectroscopy, In: MICRON41(2)pp. 130-134 PERGAMON-ELSEVIER SCIENCE LTD
    WM Tsang, V Stolojan, C Giusca, CHP Poa, B Sealy, SRP Silva, SP Wong (2006)Electron field-emission properties of Ag-SiO2 nanocomposite layers, In: JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B24(2)pp. 958-961

    In this work, Ag-Si O2 nanocomposite layers were synthesized by introducing Ag nanoclusters into thermally oxidized Si O2 layers, using ion implantation. The field-emission (FE) properties of these layers were studied and correlated with the results from atomic force microscopy and transmission electron microscopy measurements. These nanocomposites exhibit good FE properties and give an emission current of 1 nA at electric fields as low as 13 Vμm, for a dose of 5× 1016 Ag+ cm2, compared with 204 Vμm for "bare" Si O2 layers. It is clearly demonstrated that the good FE properties of these nanocomposites are attributed to two types of local-field enhancement: one due to the surface morphology and the other due to electrical inhomogeneity. The isolated conductive Ag nanoclusters embedded in the electrically insulating Si O2 matrix provide a field enhancement due to the electrical inhomogeneity effect. Moreover, the implanted Ag ions diffuse to the surface, during the implantation process, and create dense surface-protrusion structure which provides a geometric local-field enhancement. The local-field-enhancement mechanisms in these samples are critically dependent on the implantation dose of Ag. © 2006 American Vacuum Society.

    WM Tsang, AADT Adikaari, V Stolojan, SRP Silva (2008)Improving the electron emission properties of ion-beam-synthesized Ag-SiO2 nanocomposites by pulsed laser annealing, In: JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B26(2)pp. 860-863
    V Stolojan, P Moreau, MJ Goringe, SRP Silva (2006)Subnanometer-resolved measurement of the tunneling effective mass using bulk plasmons, In: APPLIED PHYSICS LETTERS88(12)ARTN 1pp. ?-? AMER INST PHYSICS

    Superlattices are periodic structures where the constituents alternate between low- and high-bandgap materials; the resulting quantum confinement tailors the resulting device properties and increases their operating speed. Amorphous carbon is an excellent candidate for both the well and barrier layers of the superlattices, leading to a fast and reliable device manufacturing process. We show theoretically and experimentally that, using low energy-loss spatially resolved spectroscopy, we can characterize the component layers of a superlattice. We measure quantum confinement of the electron wave function in the superlattice's wells and calculate the effective tunneling mass for amorphous carbon superlattices as m* =0.067 me. This effective mass makes diamondlike carbon films as feasible candidate for electronic devices. © 2006 American Institute of Physics.

    WM Tsang, V Stolojan, BJ Sealy, SRP Silva, SP Wong (2007)Electron field emission properties of co quantum dots in SiO matrix synthesised by ion implantation, In: IVNC and IFES 2006 - Technical Digest - l9th International Vacuum Nanoelectronics Conference and 50th International Field Emission Symposiumpp. 47-48 IEEE

    Crystallised Co nanoparticles were synthesized by Co+ implantation onto thermally oxidised SiO2 layers on silicon substrate. The implantation energy was 50 keV and the doses ranged from 1 to 7times1016 Co+/cm2. The possibility of controlling the size and distribution of the nanoclusters by changing implantation conditions (e.g. dose and energy) is the main advantage of this technique. Atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (X-TEM) were used to characterize the nanoclusters. The staircase I-V curve also shows that the metallic quantum dots embedded in a thin SiO2 layer on silicon substrate has effective Coulomb blockade at room temperature

    V Stolojan, P Moreau, MJ Goringe, SRP Silva, AG Cullis, JL Hutchison (2005)Quantum effects in band gap-modulated amorphous carbon superlattices, In: Microscopy of Semiconducting Materials107pp. 307-310
    Mehmet O. Tas, Mark A. Baker, Mateus G. Masteghin, Jedidiah Bentz, Keir Boxshall, Vlad Stolojan (2019)Highly Stretchable, Directionally Oriented Carbon Nanotube/PDMS Conductive Films with Enhanced Sensitivity as Wearable Strain Sensors, In: ACS Applied Materials & Interfaces American Chemical Society

    Recent interest in the fields of human motion monitoring, electronic skin, and human–machine interface technology demands strain sensors with high stretchability/compressibility (ε > 50%), high sensitivity (or gauge factor (GF > 100)), and long-lasting electromechanical compliance. However, current metal- and semiconductor-based strain sensors have very low (ε < 5%) stretchability or low sensitivity (GF < 2), typically sacrificing the stretchability for high sensitivity. Composite elastomer sensors are a solution where the challenge is to improve the sensitivity to GF > 100. We propose a simple, low-cost fabrication of mechanically compliant, physically robust metallic carbon nanotube (CNT)-polydimethylsiloxane (PDMS) strain sensors. The process allows the alignment of CNTs within the PDMS elastomer, permitting directional sensing. Aligning CNTs horizontally (HA-CNTs) on the substrate before embedding in the PDMS reduces the number of CNT junctions and introduces scale-like features on the CNT film perpendicular to the tensile strain direction, resulting in improved sensitivity compared to vertically-aligned CNT-(VA-CNT)-PDMS strain sensors under tension. The CNT alignment and the scale-like features modulate the electron conduction pathway, affecting the electrical sensitivity. Resulting GF values are 594 at 15% and 65 at 50% strains for HA-CNT-PDMS and 326 at 25% and 52 at 50% strains for VA-CNT-PDMS sensors. Under compression, VA-CNT-PDMS sensors show more sensitivity to small-scale deformation than HA-CNT-PDMS sensors due to the CNT orientation and the continuous morphology of the film, demonstrating that the sensing ability can be improved by aligning the CNTs in certain directions. Furthermore, mechanical robustness and electromechanical durability are tested for over 6000 cycles up to 50% tensile and compressive strains, with good frequency responses with negligible hysteresis. Finally, both types of sensors are shown to detect small-scale human motions, successfully distinguishing various human motions with reaction and recovery times of as low as 130 ms and 0.5 s, respectively.

    V Tison, V Stolojan, PCP Watts, DC Cox, GY Chen, SRP Silva (2008)Gas Sensing Properties of Vapour-Deposited Tungsten Oxide Nanostructures, In: MICROSCOPY OF SEMICONDUCTING MATERIALS 2007120pp. 281-284
    GY Chen, CHP Poa, Vlad Stolojan, S Silva (2006)Novel approach to low substrate temperature synthesis of carbon nanotubes, In: 2006 IEEE Conference on Emerging Technologies - Nanoelectronicspp. 254-257

    We present a novel approach, which will potentially allow for low-temperature-substrate synthesis of carbon nanotubes using direct-current plasma-enhanced chemical vapour deposition. The approach utilizes top-down plasma heating rather than conventional heating from a conventional substrate heater under the electrode. In this work, a relatively thick titanium layer is used as a thermal barrier to create a temperature gradient between the Ni catalyst surface and the substrate. We describe the growth properties as a function of the bias voltage and the hydrocarbon concentrations. The heating during growth is provided solely by the plasma, which is dependent only on the process conditions, which dictate the power density and the cooling of the substrate, plus now the thermal properties of the "barrier layer". This novel approach of using plasma heating and thermal barrier allows for the synthesis of carbon nanotubes at low substrate temperature conditions to be attained with suitable cooling schemes.

    V Stolojan, P Moreau, SJ Henley, MJ Goringe, SRP Silva (2006)Energy loss spectroscopic profiling across linear interfaces: The example of amorphous carbon superlattices, In: ULTRAMICROSCOPY106(4-5)pp. 346-355 ELSEVIER SCIENCE BV

    Energy loss spectroscopic profiling is a way to acquire, in parallel, spectroscopic information across a linear feature of interest, using a Gatan imaging filter (GIF) fitted to a transmission electron microscope (TEM). This technique is capable of translating the high spatial resolution of a bright field image into a sampling of the spectral information with similar resolution. Here we evaluate the contributions of chromatic aberration and the various acquisition parameters to the spatial sampling resolution of the spectral information, and show that this can reach 0.5 nm, in a system not ordinarily capable of forming electron probes smaller than 2 nm. We use this high spatial sampling resolution to study the plasmon energy variation across amorphous carbon superlattices, in order to extract information about their structure and electronic properties. By modelling the interaction of the relativistic incident electrons with a dielectric layer sandwiched between outer layers, we show that, due to the screening of the interfaces and at increased collection angles, the plasmon energy in the sandwiched layer can still be identified for layer thicknesses down to 5 A. This allows us to measure the change in the well bandgap as a function of well width and to interpret it in terms of the changes in the sp2 -fractions due to the deposition method, as measured from the carbon K-edges, and in terms of quantum confinement of the well wavefunction by the adjacent barriers.

    DC Cox, V Stolojan, G Chen, SR Silva, AG Cullis, PA Midgley (2008)In situ Observation of the Growth of Tungsten Oxide Nanostructures, In: MICROSCOPY OF SEMICONDUCTING MATERIALS 2007120pp. 277-280
    GY Chen, CHP Poa, SJ Henley, V Stolojan, SRP Silva, S Haq (2005)Deployment of titanium thermal barrier for low-temperature carbon nanotube growth, In: APPLIED PHYSICS LETTERS87(25)ARTN 2pp. ?-? AMER INST PHYSICS

    Chemical vapor-synthesized carbon nanotubes are typically grown at temperatures around 600 °C. We report on the deployment of a titanium layer to help elevate the constraints on the substrate temperature during plasma-assisted growth. The growth is possible through the lowering of the hydrocarbon content used in the deposition, with the only source of heat provided by the plasma. The nanotubes synthesized have a small diameter distribution, which deviates from the usual trend that the diameter is determined by the thickness of the catalyst film. Simple thermodynamic simulations also show that the quantity of heat, that can be distributed, is determined by the thickness of the titanium layer. Despite the lower synthesis temperature, it is shown that this technique allows for high growth rates as well as better quality nanotubes. © 2005 American Institute of Physics.

    YY Tan, KDGI Jayawardena, AADT Adikaari, LW Tan, JV Anguita, SJ Henley, V Stolojan, JD Carey, SRP Silva (2012)Photo-thermal chemical vapor deposition growth of graphene, In: Carbon50(2)pp. 668-673 Elsevier

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

    FC Marques, RG Lacerda, A Champi, V Stolojan, DC Cox, SRP Silva (2003)Thermal expansion coefficient of hydrogenated amorphous carbon, In: APPLIED PHYSICS LETTERS83(15)pp. 3099-3101 AMER INST PHYSICS
    V Gopee, O Thomas, C Hunt, V Stolojan, J Allam, SRP Silva (2016)Carbon Nanotube Interconnects Realized through Functionalization and Sintered Silver Attachment, In: ACS Applied Materials and Interfaces8(8)pp. 5563-5570 American Chemical Society

    Carbon nanotubes (CNTs) in the form of interconnects have many potential applications, and their ability to perform at high temperatures gives them a unique capability. We show the development of a novel transfer process using CNTs and sintered silver that offers a unique high-temperature, high-conductivity, and potentially flexible interconnect solution. Arrays of vertically aligned multiwalled carbon nanotubes of approximately 200 μm in length were grown on silicon substrates, using low-temperature photothermal chemical vapor deposition. Oxygen plasma treatment was used to introduce defects, in the form of hydroxyl, carbonyl, and carboxyl groups, on the walls of the carbon nanotubes so that they could bond to palladium (Pd). Nanoparticle silver was then used to bind the Pd-coated multiwalled CNTs to a copper substrate. The silver–CNT–silver interconnects were found to be ohmic conductors, with resistivity of 6.2 × 10–4 Ωm; the interconnects were heated to temperatures exceeding 300 °C (where common solders fail) and were found to maintain their electrical performance.

    M Palumbo, T Lutz, CE Giusca, H Shiozawa, V Stolojan, DC Cox, RM Wilson, SJ Henley, SRP Silva (2009)From Stems (and Stars) to Roses: Shape-Controlled Synthesis of Zinc Oxide Crystals, In: CRYSTAL GROWTH & DESIGN9(8)pp. 3432-3437 AMER CHEMICAL SOC
    Bobur Mirkhaydarov, Haris Votsi, Abhishek Sahu, Philippe Caroff, Paul R. Young, Vlad Stolojan, Simon King, Calvin C H Ng, Vijaya Devabhaktuni, Hoe H Tan, Chennupati Jagadish, Peter Aaen, Maxim Shkunov (2019)Solution‐Processed InAs Nanowire Transistors as Microwave Switches, In: Advanced Electronic Materials5(1)1800323 Wiley

    The feasibility of using self‐assembled InAs nanowire bottom‐gated field‐effect transistors as radio‐frequency and microwave switches by direct integration into a transmission line is demonstrated. This proof of concept is demonstrated as a coplanar waveguide (CPW) microwave transmission line, where the nanowires function as a tunable impedance in the CPW through gate biasing. The key to this switching capability is the high‐performance, low impedance InAs nanowire transistor behavior with field‐effect mobility of ≈300 cm2 V−1 s−1, on/off ratio of 103, and resistance modulation from only 50 Ω in the full accumulation mode, to ≈50 kΩ when the nanowires are depleted of charge carriers. The gate biasing of the nanowires within the CPW results in a switching behavior, exhibited by a ≈10 dB change in the transmission coefficient, S21, between the on/off switching states, over 5–33 GHz. This frequency range covers both the microwave and millimeter‐wave bands dedicated to Internet of things and 5G applications. Demonstration of these switches creates opportunities for a new class of devices for microwave applications based on solution‐processed semiconducting nanowires.

    V Stolojan (2015)Nanometrology Using the Transmission Electron Microscope Morgan & Claypool Publishers

    The Transmission Electron Microscope (TEM) is the ultimate tool to see and measure structures on the nanoscale and to probe their elemental composition and electronic structure with sub-nanometer spatial resolution. Recent technological breakthroughs have revolutionized our understanding of materials via use of the TEM, and it promises to become a significant tool in understanding biological and bio-molecular systems such as viruses and DNA molecules. This book is a practical guide for scientists who need to use the TEM as a tool to answer questions about physical and chemical phenomena on the nanoscale.

    Tingting Zhang, Emilia Olsson, Mohammadmehdi Choolaei, Vlad Stolojan, Chuanqi Feng, Huimin Wu, Shiquan Wang, Qiong Cai (2020)Synthesis and Electrochemical Properties of Bi2MoO6/Carbon Anode for Lithium-Ion Battery Application, In: Materials13(5)1132 MDPI

    High capacity electrode materials are the key for high energy density Li-ion batteries (LIB) to meet the requirement of the increased driving range of electric vehicles. Here we report the synthesis of a novel anode material, Bi2MoO6/palm-carbon composite, via a simple hydrothermal method. The composite shows higher reversible capacity and better cycling performance, compared to pure Bi2MoO6. In 0–3 V, a potential window of 100 mA/g current density, the LIB cells based on Bi2MoO6/palm-carbon composite show retention reversible capacity of 664 mAh·g−1 after 200 cycles. Electrochemical testing and ab initio density functional theory calculations are used to study the fundamental mechanism of Li ion incorporation into the materials. These studies confirm that Li ions incorporate into Bi2MoO6 via insertion to the interstitial sites in the MoO6-layer, and the presence of palm-carbon improves the electronic conductivity, and thus enhanced the performance of the composite materials.

    GL Kerr, DC Cox, V Stolojan, SRP Silva, RT Baker, G Mobus, PD Brown (2008)Characterisation of electron-beam deposited tungsten interconnects, In: EMAG: ELECTRON MICROSCOPY AND ANALYSIS GROUP CONFERENCE 2007126pp. ?-?
    WM Tsang, SJ Henley, V Stolojan, SRP Silva (2006)Negative differential conductance observed in electron field emission from band gap modulated amorphous-carbon nanolayers, In: APPLIED PHYSICS LETTERS89(19)ARTN 1pp. ?-? AMER INST PHYSICS

    Amorphous-carbon (a-C)-based quantum confined structures were synthesized by pulsed laser deposition. In these structures, electrons are confined in a few nanometer thick sp(2) rich a-C layer, which is bound by the vacuum barrier and a 3 nm thick sp(3) rich a-C base layer. In these structures anomalous field emission properties, including negative differential conductance and repeatable switching effects, are observed when compared to control samples. These properties will be discussed in terms of resonant tunneling and are of great interest in the generation and amplification of high-frequency signals for vacuum microelectronics and fast switching devices.

    WM Tsang, SJ Henley, V Stolojan, SRP Silva (2006)Negative differential conductance observed in electron field emission from band gap modulated A-C nanolayers, In: IVNC and IFES 2006 - Technical Digest - l9th International Vacuum Nanoelectronics Conference and 50th International Field Emission Symposiumpp. 199-200
    MS Saaverda, GD Sims, LN McCartney, V Stolojan, JV Anguita, YY Tan, SL Ogin, PA Smith, SRP Silva (2012)Catalysing the production of multiple arm carbon octupi nanostructures, In: Carbon50(6)pp. 2141-2146 Elsevier

    Octopus-like carbon nanofibres with leg diameters as small as 9 nm are reported, with a high yield over large areas, using a unique photo-thermal chemical vapour deposition system. The branched nature of these nanostructures leads to geometries ideal for increasing the surface area of contacts for many electronic and electrochemical devices. The manufacture of these structures involves a combination of a polyacrylonitrile/polysiloxane film covering the surface of cupronickel catalysts, supported on silicon. Acetylene is used as the carbon feedstock. High-resolution electron microscopy revealed a relationship between the geometry of the nanoparticles and the catalytic growth process, which can be tuned to maximise geometries (and therefore the surface area) and was obtained with a catalyst size of 125 nm. The technique proposed for growing these carbon octopi nanostructures is ideal to facilitate a new in situ transfer film process to place high-density carbon structures on secondary surfaces to produce high capacitance all-carbon contacts.

    JD Carey, M Ahmad, JV Anguita, V Stolojan, T Corless, SRP Silva (2020)High Quality Carbon Nanotubes on Conductive Substrates Grown at Low Temperatures University of Surrey
    Kiron Prabha Rajeev, Michael Beliatis, Stamatis Georgakopoulos, Vlad Stolojan, John Underwood, Maxim Shkunov (2019)Laser Patterned Polymer/Carbon Nanotubes Composite Electrodes for Flexible Silicon Nanowire Transistors, In: Journal of Nanoscience and Nanotechnology19(8)pp. 4765-4770

    Fabrication techniques such as laser patterning offer excellent potential for low cost and large area device fabrication. Conductive polymers can be used to replace expensive metallic inks such as silver and gold nanoparticles for printing technology. Electrical conductivity of the polymers can be improved by blending with carbon nanotubes. In this work, formulations of acid functionalized multiwalled carbon nanotubes (f-MWCNTs) and poly(ethylenedioxythiophene) [PEDOT]:polystyrene sulphonate [PSS] were processed, and thin films were prepared on plastic substrates. Conductivity of PEDOT:PSS increased almost four orders of magnitude after adding f-MWCNTs. Work function of PEDOT:PSS/f-MWCNTs films was ∼0.5 eV higher as compared to the work function of pure PEDOT:PSS films, determined by Kelvin probe method. Field-effect transistors source–drain electrodes were prepared on PET plastic substrates where PEDOT:PSS/f-MWCNTs were patterned using laser ablation at 44 mJ/pulse energy to define 36 μm electrode separation. Silicon nanowires were deposited using dielectrophoresis alignment technique to bridge laser patterned electrodes. Top-gated nanowire field effect transistors were completed by depositing parylene C as polymer gate dielectric and gold as the top-gate electrode. Transistor characteristics showed p-type conduction with excellent gate electrode coupling, with an ON/OFF ratio of ∼200. Thereby, we demonstrate the feasibility of using high workfunction, printable PEDOT:PSS/f-MWCNTs composite inks for laser patterned source/drain electrodes for nanowire transistors on flexible substrates.

    GY Chen, Vlad Stolojan, David Cox, C Giusca, S Silva (2006)Growth of tungsten oxide nanowires using simple thermal heating, In: 2006 IEEE Conference on Emerging Technologies - Nanoelectronicspp. 376-378

    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.

    Muhammad Ahmad, Jose Anguita, Vlad Stolojan, T Corless, J Chen, James Carey, S Silva (2015)High Quality Carbon Nanotubes on Conductive Substrates Grown at Low Temperatures, In: Advanced Functional Materials25(28)pp. 4419-4429 Wiley

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

    SRP Silva, V Stolojan (2005)Electron energy loss spectroscopy of carbonaceous rnaterials, In: THIN SOLID FILMS488(1-2)pp. 283-290
    Vlad Stolojan, L McCafferty, Simon King, Wei Zhang, S Haq, Ravi Silva (2015)Decoration of multiwalled carbon nanotubes with protected iron nanoparticles, In: Carbon84pp. 47-55 Elsevier

    A method to simultaneously synthesize carbon-encapsulated magnetic iron nanoparticles (Fe-NPs) and attach these particles to multi-walled carbon nanotubes (MWCNT) is presented. Thermal decomposition of cyclopentadienyliron dicarbonyl dimer [(C5H5)(2)Fe-2(CO)(4)], over a range of temperatures from 250 degrees C to 1200 degrees C, results in the formation of Fe-NPs attached to MWCNT. At the same time, a protective carbon shell is produced and surrounds the Fe-NPs, covalently attaching the particles to the MWCNT and leading to resistance to acid dissolution. The carbon coating varies in degree of graphitisation, with higher synthesis temperatures leading to a higher degree of graphitisation. The growth model of the nanoparticles and subsequent mechanism of MWCNT attachment is discussed. Adsorption potential of the hybrid material towards organic dyes (Rhodamine B) has been displayed, an indication of potential uses as a material for water treatment. The material has also been electrospun into aligned nanocomposite fibres to produce a soft magnetic composite (SMC) with future applications in sensors and fast switching solenoids.

    V Stolojan, MJ Whiting, MJ Goringe, MJ Kelly, SRP Silva (2003)Electron energy loss line spectral and TEM analysis of heterojunctions, In: MICROSCOPY OF SEMICONDUCTING MATERIALS 2003(180)pp. 41-44
    GY Chen, CHP Poa, SJ Henley, V Stolojan, SRP Silva, S Haq (2005)Deployment of titanium thermal barrier for low-temperature carbon nanotube growth, In: APPLIED PHYSICS LETTERS87(25)ARTN 2pp. ?-? AMER INST PHYSICS

    Chemical vapor-synthesized carbon nanotubes are typically grown at temperatures around 600 degrees C. We report on the deployment of a titanium layer to help elevate the constraints on the substrate temperature during plasma-assisted growth. The growth is possible through the lowering of the hydrocarbon content used in the deposition, with the only source of heat provided by the plasma. The nanotubes synthesized have a small diameter distribution, which deviates from the usual trend that the diameter is determined by the thickness of the catalyst film. Simple thermodynamic simulations also show that the quantity of heat, that can be distributed, is determined by the thickness of the titanium layer. Despite the lower synthesis temperature, it is shown that this technique allows for high growth rates as well as better quality nanotubes.

    BO Boskovic, V Stolojan, DA Zeze, RD Forrest, SRP Silva, S Haq (2004)Branched carbon nanofiber network synthesis at room temperature using radio frequency supported microwave plasmas, In: JOURNAL OF APPLIED PHYSICS96(6)pp. 3443-3446 AMER INST PHYSICS
    W Zhang, V Stolojan, SRP Silva, CW Wu (2014)Raman, EELS and XPS studies of maghemite decorated multi-walled carbon nanotubes, In: SPECTROCHIMICA ACTA PART A-MOLECULAR AND BIOMOLECULAR SPECTROSCOPY121pp. 715-718 PERGAMON-ELSEVIER SCIENCE LTD
    Simon G. King, Nicholas J. Terrill, Andrew J. Goodwin, Robert Stevens, Vlad Stolojan, Ravi P. Silva (2018)Probing of Polymer to Carbon Nanotube Surface Interactions within Highly Aligned Electrospun Nanofibers for Advanced Composites, In: Carbon138pp. 207-214 Elsevier

    By electrospinning poly(ethylene oxide) (PEO)-blended sodium dodecyl sulfate (SDS) functionalized carbon nanotube (CNT) solutions, we engineered single- and double-walled nanotubes into highly aligned arrays. CNT alignment was measured using electron microscopy and polarised Raman spectroscopy. Mechanical tensile testing demonstrates that a CNT loading of 3.9wt% increases the ultimate tensile strength and ductility of our composites by over a factor of 3, and the Young's modulus by over a factor of 4, to ∼260MPa. Transmission electron microscopy (TEM) reveals how the aligned nanotubes provide a solid structure, preventing polymer chains from slipping, as well as polymer crystallisation structures such as ‘shish-kebabs’ forming, which are responsible for the improved mechanical properties of the composite. Differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS) reveals micellar and hexagonal columnar structures along the axis of the fibers, some of which are associated with the presence of the CNT, where these hexagonal structures are associated with the SDS functionalization on the CNT surfaces. This work demonstrates the benefits of CNT alignment within composites, revealing the effectiveness of the electrospinning technique, which enables significantly improved functionality, increasing the utility of the composites for use in many different technological areas.

    WM Tsang, V Stolojan, C Giusca, CHP Poa, B Sealy, SRP Silva, SP Wong (2005)The electron field emission properties of Ag-SiO2 nanocomposite layers, In: Technical Digest of the 18th International Vacuum Nanoelectronics Conference, IVNC 20052005pp. 206-207
    H Shiozawa, Anne Skeldon, David Lloyd, V Stolojan, DC Cox, SR Silva (2011)Spontaneous emergence of long-range shape symmetry, In: Nano Letters11(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.

    Marios Constantinou, Kai Hoettges, S Krylyuk, MB Katz, A Davydov, Grigorios Rigas, Vlad Stolojan, Michael Hughes, Maxim Shkunov (2017)Rapid determination of nanowires electrical properties using a dielectrophoresis-well based system, In: Applied Physics Letters110(13) AIP Publishing.

    The use of high quality semiconducting nanomaterials for advanced device applications has been hampered by the unavoidable growth variability of electrical properties of one-dimensional nanomaterials, such as nanowires and nanotubes, thus highlighting the need for the characterization of efficient semiconducting nanomaterials. In this study, we demonstrate a low-cost, industrially scalable dielectrophoretic (DEP) nanowire assembly method for the rapid analysis of the electrical properties of inorganic single crystalline nanowires, by identifying key features in the DEP frequency response spectrum from 1 kHz to 20 MHz in just 60 s. Nanowires dispersed in anisole were characterized using a three-dimensional DEP chip (3DEP), and the resultant spectrum demonstrated a sharp change in nanowire response to DEP signal in 1–20 MHz frequency range. The 3DEP analysis, directly confirmed by field-effect transistor data, indicates that nanowires of higher quality are collected at high DEP signal frequency range above 10 MHz, whereas lower quality nanowires, with two orders of magnitude lower current per nanowire, are collected at lower DEP signal frequencies. These results show that the 3DEP platform can be used as a very efficient characterization tool of the electrical properties of rod-shaped nanoparticles to enable dielectrophoretic selective deposition of nanomaterials with superior conductivity properties.

    Mateus G. Masteghin, Muhammad Ahmad, Mehmet Tas, Christopher Smith, Vlad Stolojan, David Cox, Ravi Silva (2020)Field electron emission measurements as a complementary technique to assess carbon nanotube quality, In: Applied physics letters116103101 American Institute of Physics

    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 (3070) results when a lower ID/IG is present (0.45), with initial emissions at larger distances (47 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.

    RE Waters, Mark Whiting, Vlad Stolojan (2013)Examining the Pearlite Growth Interface in a Fe-C-Mn Alloy, In: International Journal of Materials and Metallurgical Engineering7(7)pp. 506-509 World Academy of Science, Engineering and Technology

    A method of collecting composition data and examining structural features of pearlite lamellae and the parent austenite at the growth interface in a 13wt. % manganese steel has been demonstrated with the use of Scanning Transmission Electron Microscopy (STEM). The combination of composition data and the structural features observed at the growth interface show that available theories of pearlite growth cannot explain all the observations.

    RA Hatton, NP Blanchard, V Stolojan, AJ Miller, SRP Silva (2007)Nanostructured copper phthalocyanine-sensitized multiwall carbon nanotube films, In: LANGMUIR23(11)pp. 6424-6430 AMER CHEMICAL SOC
    Vlad Stolojan (2019)Textile-compatible, electroactive Polyvinylidene fluoride electrospun mats for energy harvesting, In: Macromolecular Chemistry and Physics Wiley
    RG Lacerda, V Stolojan, DC Cox, SRP Silva, FC Marques (2002)Structural characterization of hard a-C : H films as a function of the methane pressure, In: DIAMOND AND RELATED MATERIALS11(3-6)pp. 980-984
    Y Tison, CE Giusca, V Stolojan, Y Hayashi, SRP Silva (2008)The inner shell influence on the electronic structure of double-walled carbon nanotubes, In: ADVANCED MATERIALS20(1)pp. 189-+ WILEY-V C H VERLAG GMBH
    Y Hayashi, T Tokunaga, K Kaneko, Simon Henley, Vlad Stolojan, James Carey, SRP Silva (2006)Microstructure analyses of metal-filled carbon nanotubes synthesized by microwave plasma-enhanced chemical vapor deposition, In: IEEE TRANSACTIONS ON NANOTECHNOLOGY5(5)pp. 485-490 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

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

    WM Tsang, V Stolojan, SP Wong, BJ Sealy, SRP Silva (2005)The electron field emission properties of ion beam synthesised metal-dielectric nanocomposite layers on silicon substrates, In: MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY124pp. 453-457
    Christopher Smith, Christopher A. Mills, Silvia Pani, Rhys Rhodes, Josh J. Bailey, Samuel J. Cooper, Tanveerkhan S. Pathan, Vlad Stolojan, Daniel J. L. Brett, Paul R. Shearing, S. Ravi P. Silva (2019)X-ray micro-computed tomography as a non-destructive tool for imaging the uptake of metal nanoparticles by graphene-based 3D carbon structures, In: Nanoscale11pp. 14734-14741 Royal Society of Chemistry

    Graphene-based carbon sponges can be used in different applications in a large number of fields including microelectronics, energy harvesting and storage, antimicrobial activity and environmental remediation. The functionality and scope of their applications can be broadened considerably by the introduction of metallic nanoparticles into the carbon matrix during preparation or post-synthesis. Here, we report on the use of X-ray micro-computed tomography (CT) as a method of imaging graphene sponges after the uptake of metal (silver and iron) nanoparticles. The technique can be used to visualize the inner structure of the graphene sponge in 3D in a non-destructive fashion by providing information on the nanoparticles deposited on the sponge surfaces, both internal and external. Other deposited materials can be imaged in a similar manner providing they return a high enough contrast to the carbon microstructure, which is facilitated by the low atomic mass of carbon.

    CE Giusca, V Stolojan, J Sloan, F Börrnert, H Shiozawa, K Sader, MH Rümmeli, B Büchner, SR Silva (2013)Confined crystals of the smallest phase-change material., In: Nano Lett13(9)pp. 4020-4027

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

    BO Boskovic, V Stolojan, RUA Khan, S Haq, SRP Silva (2002)Large-area synthesis of carbon nanofibres at room temperature, In: NATURE MATERIALS1(3)pp. 165-168 NATURE PUBLISHING GROUP
    MA Monclus, MA Baker, C Rebhoz, V Stolojan, PN Gibson, A Leyland, A Matthews (2006)Nanostructural studies of PVD TiAlB coatings, In: SURFACE AND INTERFACE ANALYSIS38(4)pp. 731-735
    V Stolojan, SRP Silva, MJ Goringe, RLD Whitby, WK Hsu, DRM Walton, HW Kroto (2005)Dielectric properties of WS2-coated multiwalled carbon nanotubes studied by energy-loss spectroscopic profiling, In: APPLIED PHYSICS LETTERS86(6)ARTN 0pp. ?-? AMER INST PHYSICS
    Y Hayashi, T Tokunaga, K Kaneko, SJ Henley, V Stolojan, JD Carey, SRP Silva (2006)Microstructure analyses of metal-filled carbon nanotubes synthesized by microwave plasma-enhanced chemical vapour deposition, In: 2006 IEEE Conference on Emerging Technologies - Nanoelectronicspp. 93-98