Browsing by Author "Kodu, Margus"

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Metal Oxide Nanolayer-Decorated Epitaxial Graphene: A Gas Sensor Study
(MDPI, 2020) Rodner, Marius; Icardi, Adam; Kodu, Margus; Jaaniso, Raivo; Schütze, Andreas; Eriksson, Jens
In this manuscript, we explore the sensor properties of epitaxially grown graphene on silicon carbide decorated with nanolayers of CuO, Fe3O4, V2O5, or ZrO2. The sensor devices were investigated in regard to their response towards NH3 as a typical reducing gas and CO, C6H6, CH2O, and NO2 as gases of interest for air quality monitoring. Moreover, the impact of operating temperature, relative humidity, and additional UV irradiation as changes in the sensing environment have been explored towards their impact on sensing properties. Finally, a cross-laboratory study is presented, supporting stable sensor responses, and the final data is merged into a simplified sensor array. This study shows that sensors can be tailored not only by using different materials but also by applying different working conditions, according to the requirements of certain applications. Lastly, a combination of several different sensors into a sensor array leads to a well-performing sensor system that, with further development, could be suitable for several applications where there is no solution on the market today.
Pulsed Laser Deposition of Magnesium Oxide and Barium Ternary Oxides for Plasma Display Protective Layers
(2011-06-10) Kodu, Margus
Antud doktoritöös uuriti plasmaekraanide kaitsekihtide materjale ühe plasmaekraanide olulisima parameetri, süttimispinge, seisukohast. Plasmaekraanid koosnevad tuhandtetest gaaslahendusrakkudest ja ekraanide energiatarve ja maksumus on otseselt seotud rakkudes tekitatava gaaslahenduse süttimispingega - mida madalam süttimispinge, seda madalam energiatarve ja seda vähem maksavad ekraanis sisalduvad elektroonikakomponendid. Sünteesitud materjalide süttimispingete mõõtmiseks sadestati uuritavast materjalist kiled spetsiaalsetele alektroodalustele ja mõõdeti kahest vastakuti asetatud alusest koosneva testraku süttimispinge. Õhukeste kilede sadesamiseks kasutati impulss-lasersadestuse seadet. Töös käsitletud materjalideks olid BaGa2O4, BaY2O4 ning puhas ja dopeeritud MgO. Magneesium oksiidi - mis on ka hetkel plasmaekraanis tööstuslikult kasutatav kaitsekihi materjal - korral uuriti sadestustingimuste mõju kasvanud kilede struktuurile ning kilede struktuuri ja dopeerimise mõju gaaslahendusomadustele. Uuriti ka kahe potentsiaalse kaitsekihi asendusmaterjali, BaGa2O4 ja BaY2O4, kilede struktuuri ja gaaslahendusomadusi. Olulisimad faktorid madala süttimispinge saavutamiseks olid MgO kilede korral nende tihedus ja kristallilisus - mida suurem oli kilede tihedus ja kristallilisus, seda väiksem oli süttimispinge. Lisaks selgus analüüsiandmetest, et ka kilede suur pinnakaredus soodustab madala süttimispinge saavutamist. Doktoritöös sünteesiti ka vesinikuga dopeeritud MgO õhukesed kiled. Analüüs näitas, et vesinikulisand tekitas magnesium oksiidi kristallis defekte, mis on võimelised lõksustama elektrone. Need elektronlõksud mõjutasid kilede gaaslahendusomadusi nii, et dopeeritud kilede süttimispinge oli kuni 55 V (20%) madalam kui vesinikulisandita kilede süttimispinge. Kahe uuritud baariumi kolmikoksiidi, BaY2O4 ja BaGa2O4, korral saadi süttimispingete väärtusteks 210 V (BaY2O4) ja 257 V (BaGa2O4), mis on oluliselt kõrgemad võrreldes MgO kiledel mõõdetud väärtustega (<180 V). Tulemus viitab sellele, et need baariumi ühendid pole sobivad MgO asendusmaterjalid kuna nende kasutamine plasmakuvaris ei soodustaks seadme energiatarbe alanemist.
Semiquantitative Classification of Two Oxidizing Gases with Graphene-Based Gas Sensors
(MDPI, 2022) Lind, Martin; Kiisk, Valter; Kodu, Margus; Kahro, Tauno; Renge, Indrek; Avarmaa, Tea; Makaram, Prashanth; Zurutuza, Amaia; Jaaniso, Raivo
Miniature and low-power gas sensing elements are urgently needed for a portable electronic nose, especially for outdoor pollution monitoring. Hereby we prepared chemiresistive sensors based on wide-area graphene (grown by chemical vapor deposition) placed on Si/Si3N4 substrates with interdigitated electrodes and built-in microheaters. Graphene of each sensor was individually functionalized with ultrathin oxide coating (CuO-MnO2, In2O3 or Sc2O3) by pulsed laser deposition. Over the course of 72 h, the heated sensors were exposed to randomly generated concentration cycles of 30 ppb NO2, 30 ppb O3, 60 ppb NO2, 60 ppb O3 and 30 ppb NO2 + 30 ppb O3 in synthetic air (21% O2, 50% relative humidity). While O3 completely dominated the response of sensors with CuO-MnO2 coating, the other sensors had comparable sensitivity to NO2 as well. Various response features (amplitude, response rate, and recovery rate) were considered as machine learning inputs. Using just the response amplitudes of two complementary sensors allowed us to distinguish these five gas environments with an accuracy of ~ 85%. Misclassification was mostly due to an overlap in the case of the 30 ppb O3, and 30 ppb O3 + 30 ppb NO2 responses, and was largely caused by the temporal drift of these responses. The addition of recovery rates to machine learning input variables enabled us to very clearly distinguish different gases and increase the overall accuracy to ~94%.