Graphene functionalised by laser ablated V2O5 as highly sensitive NH3 sensor

Graphene has been recognized as a promising gas sensing material. The response of graphene-based sensors can be radically improved by introducing defects in graphene using, e. g., metal or metal oxide nanoparticles. We have functionalised CVD grown, single layer graphene by applying pulsed laser deposition (PLD) of V2O5 which resulted in a thin V2O5 layer on graphene with average thickness of ~0.6 nm. According to Raman analysis, PLD process also induced defects in graphene. Compared to unmodified graphene, the obtained chemiresistive sensor showed considerable improvement of sensing ammonia at room temperature. In addition, also the response time, sensitivity and reversibility were essentially enhanced due to graphene functionalisation by laser deposited V2O5. This can be explained by increased surface density of gas adsorption sites introduced by high energy atoms in laser ablation plasma and formation of nanophase boundaries between deposited V2O5 and graphene.Comment: 22 pages, 6 figure

Structure-Dependent CO 2

Rare earth oxycarbonates are potential candidate materials for constructing simple and low-cost chemiresistive sensors for monitoring carbon dioxide (CO2) gas in the living and working environment for personal comfort and health reasons. Also, measurement of CO2 concentrations is needed in many industrial processes. Specifically, sol-gel made nanoparticles of Nd and La oxycarbonates have been studied previously as novel CO2 gas sensor materials. In this paper, pulsed laser deposition of La oxycarbonate (La2O2CO3) thin films was studied and structural properties of obtained thin films were characterized. Also, CO2 gas sensing ability of synthesized films was evaluated. The films deposited under CO2 partial pressure in various conditions were all Raman amorphous. In situ or ex situ annealing procedure at high CO2 partial pressure was needed for obtaining crystalline La2O2CO3 films, whereby hexagonal and monoclinic polymorphs were obtained in ex situ and in situ processes, respectively. Sensor structure, made using in situ process, was sensitive to CO2 gas and showed relatively fast response and recovery characteristics

Semiquantitative Classification of Two Oxidizing Gases with Graphene-Based Gas Sensors

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%

Magneesium oksiidi ja baariumi kolmikoksiidide õhukeste kilede impulss-lasersadestamine plasmakuvarite kaitsekihtide rakenduseks

Väitekirja elektooniline versioon ei sisalda publikatsioone.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.Plasma displays and liquid crystal displays are currently two main large screen display technologies oriented to home-consumers’ market. Besides other elements that influence the PDP characteristics most, the thin layer (protective layer) of MgO in plasma screen is one key element in PDPs and its properties have large influence on the display’s image quality, its lifetime, cost and power consumption. In this work, the morphological and defect structure of MgO thin films was modified by varying film synthesizing conditions. The influence of these modifications on films’ characteristics was studied from the standpoint of PDP applications. Some possible replacement materials for MgO were also investigated. The main attention was turned to the materials group of barium ternary oxides. The thin films, investigated in this work, were synthesized by the pulsed laser deposition (PLD) method. The firing voltage (FV) of the deposited samples, which is the direct indicator of the power consumption of the PDP device (low FV means low power consumption), was measured in our experiments. The density and the crystallinity of the MgO films were the main factors influencing the FV values – high density and crystallinity was accompanied by low FV values of the samples. It was also observed that the high surface roughness of the films favored the achieving of low FVs. MgO films with hydrogen impurities were synthesized. Analysis indicated that the hydrogen impurities in MgO crystal created defects capable of catching electrons. These electron traps influenced the electronic properties of the thin films so that they had up to 55 V (27%) lower FVs than the ones without the hydrogen impurities. Two barium ternary oxides, BaY2O4 and BaGa2O4, were considered as a plasma display protective coating material candidates. The FV values of BaY2O4 (210 V) and BaGa2O4 (257 V) were considerably higher as compared to the FVs of MgO films (<180 V), which indicates that these barium oxides are not promising replacement materials to MgO. Because of considerably higher FVs, these materials would not favor lower power consumption of a PDP device

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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

Gas-Sensing Properties of Graphene Functionalized with Ternary Cu-Mn Oxides for E-Nose Applications

Chemiresistive gas sensors were produced by functionalizing graphene with a ~3 nm layer of mixed oxide xCu2O⸱yMnO using pulsed laser deposition (PLD) from a hopcalite CuMn2O4 target. Sensor response time traces were recorded for strongly oxidizing (NO2, O3) and reducing (NH3, H2S) poisonous gases at ppb and ppm levels, respectively. The morphology of the MOX layer was modified by growth temperature during PLD, resulting in the optimization of the sensor response. Differences in decomposition or oxidation rates on catalytically active metal oxide (MOX) were utilized to achieve partial selectivity for pairs of gases that have similar adsorption and redox properties. The predominant selectivity towards ozone in most samples at different measuring conditions remained difficult to suppress. A distinct selectivity for H2S emerged at higher measurement temperatures (100–150 °C), which was assigned to catalytic oxidation with O2. Several gas–MOX interaction mechanisms were advanced to tentatively explain the sensor behavior, including reversible electron transfer in the simplest case of NO2, decomposition via ionic transients for O3, and complex catalytic oxidative transformations for NH3 and H2S

Graphene-Based Ammonia Sensors Functionalised with Sub-Monolayer V₂O₅: A Comparative Study of Chemical Vapour Deposited and Epitaxial Graphene <xref rid="fn1-sensors-429828" ref-type="fn">†</xref>

Graphene in its pristine form has demonstrated a gas detection ability in an inert carrier gas. For practical use in ambient atmosphere, its sensor properties should be enhanced with functionalisation by defects and dopants, or by decoration with nanophases of metals or/and metal oxides. Excellent sensor behaviour was found for two types of single layer graphenes: grown by chemical vapour deposition (CVD) and transferred onto oxidized silicon (Si/SiO2/CVDG), and the epitaxial graphene grown on SiC (SiC/EG). Both graphene samples were functionalised using a pulsed laser deposited (PLD) thin V2O5 layer of average thickness &#8776; 0.6 nm. According to the Raman spectra, the SiC/EG has a remarkable resistance against structural damage under the laser deposition conditions. By contrast, the PLD process readily induces defects in CVD graphene. Both sensors showed remarkable and selective sensing of NH3 gas in terms of response amplitude and speed, as well as recovery rate. SiC/EG showed a response that was an order of magnitude larger as compared to similarly functionalised CVDG sensor (295% vs. 31% for 100 ppm NH3). The adsorption site properties are assigned to deposited V2O5 nanophase, being similar for both sensors, rather than (defect) graphene itself. The substantially larger response of SiC/EG sensor is probably the result of the smaller initial free charge carrier doping in EG

Near ambient pressure X-ray photoelectron - and impedance spectroscopy study of NiO - Ce 0.9 Gd 0.1 O 2-δ anode reduction using a novel dual-chamber spectroelectrochemical cell

This paper reports an experimental study, where the reduction of NiO-GDC to Ni-GDC is monitored using high temperature (HT) near ambient pressure (NAP) X-ray photoelectron spectroscopy (XPS) in combination with simultaneous impedance spectroscopy (IS). The experiment is carried out using a dual chamber (DC)-HT-NAP-XPS cell designed for membrane electrode studies. The dual chamber measurement cell enables adequate electrochemical feedback, i.e. the possibility of measuring the working electrode potential against a Pt reference electrode in a well-known environment (pO2 = 0.2 bar) and of monitoring the pO2 value on the studied electrode through open circuit voltage (OCV) measurements. Simultaneous changes of electrode impedance and O 1s, Ce 4d and Ni 3p electron binding energies are observed and discussed. The shape of the O 1s photoelectron peak is significantly influenced by the equilibria of Ce4+ ↔ Ce3+ and Ni2+ ↔ Ni0, influenced by pO2, in analysis chamber