Browsing by Author "Kivioja, Jani"

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  • Characterization of Self-Assembled Protein Monomolecular Layer and Protein-Substrate Interface
    (2009) Kainlauri, Markku
     Helsinki University of Technology | Master's thesis
    In this Master's thesis, we studied the self-assembly of a hydrophobin protein named HFBI and the possibility to use directed self-assembly with silicon surfaces. Additionally, a generic silicon template was designed and fabricated in order to enable electrical characterization of self-assembled hydrophobin monolayers in a controlled conguration setup. The directed self-assembly of HFBI protein monolayer was studied on a patterned Si{SiO2 surface. The monolayer of a protein variant (nCys-HFBI) was transferred onto the substrate after an HF dip. The sample was next immersed in a solution containing 15 nm Au nanoparticles, which were known to bind to the cystein terminal of the protein. The outcome was investigated using Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM). A clear contrast in protein adhesion and non-adhesion was observed between silicon and silicon dioxide areas, respectively. This was a new result for the patterning of HFBI protein monolayers. Electrical characterization of the Au-nanoparticle covered nCys-HFBI on top of Highly Oriented Pyrolytic Graphite (HOPG) was performed using Conductive Atomic Force Microscopy (C-AFM). The measured conductance map corresponded with the internanoparticle distance observed in AFM and SEM images. Additionally, a thousand fold dirence in conductivity was measured between plain protein monolayer and protein monolayer with 15 nm Au nanoparticles. Part of this work included the design and fabrication of a generic template for silicon{hydrophobin interface studies. The process consisted of the fabrication of a silicon template and a stencil mask, which was used as a hard mask during the evaporation of Au contacts on top of the protein monolayer. Electrical transport measurements were carried out on the nal device at room temperature and at cryogenic temperatures (77 K and 4 K). The results indicated that the protein{silicon interface device is feasible to produce and, additionally, electrical transport measurements of the self-assembled hydrophobin monolayer can be performed with the device.
  • GaN:ssä sekä AIGaN:ssä esiintyvien hilavirheiden tutkiminen positroniannihilaatiospektroskopialla
    (2000) Kivioja, Jani
     Helsinki University of Technology | Master's thesis
    Työssä tutkittiin GaN- ja AlGaN-ohutkalvonäytteissä esiintyviä hilavirheitä positroniannihilaatiospektroskopialla. Työssä käytettiin TKK:n Fysiikan laboratorion kahta hitaiden positronien suihkua ja positronien elinaikamittalaitteistoa. Työssä mitattiin positronien keskimääräinen elinaika kahdessa HVPE-kasvatetussa GaN-ohutkalvonäytteessä. Vapaan positronin elinajaksi GaN-hilassa saatiin 156 ps ja galliumvakanssiin loukkuuntuneen positronin elinajaksi 235 ps. Nämä ovat samat kuin aiemmin tutkituissa erilliskiteissä. Näytteiden ei havaittu sisältävän Ga-vakanssien lisäksi muita positroneja loukkuunnuttavia virheitä. Positronin loukkuuntumiskertoimen lämpötilariippuvuus määritettiin elinaikamittausten perusteella. Loukkuuntumiskertoimen havaittiin noudattavan T[1/2]-lämpötilariippuvuutta, mikä osoittaa galliumvakanssin olevan negatiivisesti varautunut. Mitattujen GaN-näytteiden dislokaatio- ja epäpuhtaustiheyden oli mitattu olevan suurempi lähellä substraatin ja ohutkalvon välistä rajapintaa. Mittauksissa havaittiin myös vakanssikonsentraation suurenevan, kun etäisyys GaN/Al_(2)O_(3)-rajapintaan pieneni. Galliumvakanssin ja happiepäpuhtausatomin muodostaman (V_(Ga)-O_(N))-virhekompleksin muodostumisenergia on pienempi kuin yksittäisen Ga-vakanssin. Työssä tutkittiin Ga-vakanssia ympäröivien atomien elektronien liikemääräjakaumaa. Tulosten perusteella saatiin viitteitä (V_(Ga)-O_(N))-kompleksin muodostumisesta paljon happea sisältävissä näytteissä. Tutkituissa AlGaN-näytteissä oli aiemmin havaittu DX-hilavirhe. Tätä kuvaavan atomimallin mukaan DX-keskukseen liittyy voimakas hilarelaksaatio, jonka seurauksena hilaan syntyy vakanssin tyyppinen hilavirhe. DX-keskus on mahdollista virittää joko sopivalla valolla tai termisesti, jolloin virheeseen liittyvän tyhjän tilan tulee mallin mukaan kadota. Mittauksissa pyrittiin selvittämään, voidaanko tätä havaita positronien avulla. Työssä ei kuitenkaan saatu selkeää todistetta positronien loukkuuntumisesta DX-virheisiin.
  • Evidence of Cooper-pair pumping with combined flux and voltage control
    (2005-01-26) Niskanen, Antti O.; Kivioja, Jani; Seppä, Heikki; Pekola, Jukka
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have experimentally demonstrated pumping of Cooper pairs in a single-island mesoscopic structure. The island was connected to leads through SQUID (superconducting quantum interference device) loops. Synchronized flux and voltage signals were applied whereby the Josephson energies of the SQUIDs and the gate charge were tuned adiabatically. From the current-voltage characteristics one can see that the pumped current increases in 1e steps which is due to quasiparticle poisoning on the measurement time scale, but we argue that the transport of charge is due to Cooper pairs.
  • Mesoscopic superconducting tunnel junction devices : experimental studies of performance limitations
    (2005-06-09) Kivioja, Jani
     Doctoral dissertation (article-based)
    In this work four different mesoscopic superconducting devices have been experimentally studied: an ammeter based on a hysteretic Josephson junction switching from the superconducting state to the normal state, a conventional Cooper pair pump (CPP) based on two superconducting islands separated by tunnel junctions, a novel flux assisted Cooper pair pump and a thermometer based on a tunnel junction between a superconductor and a normal metal. These devices make use of phenomena related to superconductivity and are also benefiting from some properties of small structures, e.g., of quantum mechanical tunneling and of transport of single electrons. The present research focused on investigating performance limitations of these devices and the aim was to improve their performance. A hysteretic Josephson junction switching can be used as a sensitive current threshold detector. For many purposes, a smaller critical current Ic of the detecting junction would yield a higher current sensitivity. It was found in this Thesis that the main limitation of the ammeter is that with increasing sensitivity the negative effects due to dissipation start to dominate. In this Thesis the Josephson junction was also demonstrated as a shot-noise detector, and it was concluded that this kind of a device could probably be made into an absolute on-chip detector of Fano-factors and noise in general. A charge pump is a device which can periodically transfer a discrete amount of charge Q through an electrical circuit. The measured double island CPP introduced a frequency dependent current with a value close to the expected one at low pumping frequencies. The drawback in the measurements was that the direction of the pumped current was set by the bias voltage. The results of the measurement on flux assisted pumping were indeed very promising and the studied device was able to produce sufficiently high pumped currents of around 0.1 nA with reasonable accuracy. Thermometer based on NIS (Normal metal - Insulator - Superconductor) tunnel junctions is a very promising temperature sensor for bolometric radiation sensor applications. This work showed that it can be used as a sensitive thermometer up to MHz frequencies range and that its response time is limited by the electron-phonon scattering rate.
  • Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems
    (2015) Ferrari, A.C.; Bonaccorso, F.; "Fal'ko", V.; Novoselov, K.S.; Roche, S.; Bøggild, P.; Borini, S.; Koppens, F.H.L.; Palermo, V.; Pugno, N.; Garrido, J.A.; Sordan, R.; Bianco, A.; Ballerini, L.; Prato, M.; Lidorikis, E.; Kivioja, Jani; Marinelli, C.; Ryhänen, Tapani; Morpurgo, A.; Coleman, J.N.; Nicolosi, V.; Colombo, Luigi; Fert, A.; Garcia-Hernandez, M.; Bachtold, A.; Schneider, G.F.; Guinea, F.; Dekker, C.; Barbone, M.; Sun, Zhipei; Galiotis, C.; Grigorenko, A.N.; Konstantatos, G.; Kis, A.; Katsnelson, M.; Vandersypen, L.; Loiseau, A.; Morandi, V.; Neumaier, D.; Treossi, E.; Pellegrini, V.; Polini, M.; Tredicucci, A.; Williams, G.M.; Hee Hong, B.; Ahn, J.-H.; Kim, J.M.; Zirath, H.; van Wees, B.J.; van der Zant, H.; Occhipinti, L.; Di Matteo, A.; Kinloch, I.A.; Seyller, T.; Quesnel, E.; Feng, Xinliang; Teo, K.; Rupesinghe, N.; Hakonen, Pertti; Neil, S.R.T.; Tannock, Q.; Löfwander, T.; Kinaret, Jari
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä