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  • 1.
    Bergsten, Tobias
    et al.
    RISE - Research Institutes of Sweden (2017-2019), Säkerhet och transport, Mätteknik.
    Eklund, Gunnar
    RISE - Research Institutes of Sweden (2017-2019), Säkerhet och transport, Mätteknik.
    He, Hans
    Chalmers University of Techology, Sweden.
    Comparing GaAs and graphene QHR standards for resistance realisation at SP2016Ingår i: Conference on Precision Electromagnetic Measurements 2016 (CPEM 2016), 2016Konferensbidrag (Övrigt vetenskapligt)
    Ladda ner fulltext (pdf)
    Poster
  • 2. Cherednichenko, S.
    et al.
    Danilov, A.
    Golubev, D.
    He, Hans
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik. Chalmers University of Technology, Sweden.
    Kim, K. H.
    Yakimova, R.
    Lombardi, F.
    Bauch, T.
    Lara-Avila, S.
    Kubatkin, S.
    Quantum transport at Dirac point enables graphene for terahertz heterodyne astronomy2019Ingår i: ISSTT 2019 - 30th International Symposium on Space Terahertz Technology, Proceedings BookPages 236, International Symposium on Space Terahertz Technology , 2019Konferensbidrag (Refereegranskat)
  • 3.
    He, Hans
    et al.
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik.
    Cedergren, Karin
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik.
    Shetty, Naveen
    Chalmers University of Technology, Sweden.
    Lara-Avila, Samuel
    Chalmers University of Technology, Sweden; NPL, UK.
    Kubatkin, Sergey
    Chalmers University of Technology, Sweden.
    Bergsten, Tobias
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik.
    Eklund, Gunnar
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik.
    Accurate graphene quantum Hall arrays for the new International System of Units2022Ingår i: Nature Communications, E-ISSN 2041-1723, Vol. 13, nr 1, artikel-id 6933Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Graphene quantum Hall effect (QHE) resistance standards have the potential to provide superior realizations of three key units in the new International System of Units (SI): the ohm, the ampere, and the kilogram (Kibble Balance). However, these prospects require different resistance values than practically achievable in single graphene devices (~12.9 kΩ), and they need bias currents two orders of magnitude higher than typical breakdown currents IC ~ 100 μA. Here we present experiments on quantization accuracy of a 236-element quantum Hall array (QHA), demonstrating RK/236 ≈ 109 Ω with 0.2 part-per-billion (nΩ/Ω) accuracy with IC ≥ 5 mA (~1 nΩ/Ω accuracy for IC = 8.5 mA), using epitaxial graphene on silicon carbide (epigraphene). The array accuracy, comparable to the most precise universality tests of QHE, together with the scalability and reliability of this approach, pave the road for wider use of graphene in the new SI and beyond. © 2022, The Author(s).

    Ladda ner fulltext (pdf)
    fulltext
  • 4.
    He, Hans
    et al.
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik.
    Janssen, T. J. B. M.
    NPL National Physical Laboratory, UK.
    Rozhko, S.
    NPL National Physical Laboratory, UK.
    Tzalenchuk, A.
    NPL National Physical Laboratory, UK; University of London, UK.
    Lara-Avila, S.
    Chalmers University of Technology, Sweden.
    Yakimova, R.
    Linköping University, UK.
    Kubatkin, S.
    Chalmers University of Technology, Sweden.
    Fabrication of graphene quantum hall resistance standard in a cryogen-Table-Top system2016Ingår i: CPEM 2016 - Conference on Precision Electromagnetic Measurements, Conference DigestArtikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have demonstrated quantum Hall resistance measurements with metrological accuracy in a relatively easy to use and compact cryogen-free system operating at a temperature of around 3.8 K and magnetic field below 5 T. This advance in technology is due to the unique properties of epitaxial graphene on silicon carbide (SiC) which lifts the stringent requirements for quantum hall effect seen in conventional semiconductors. This paper presents the processes involved in fabrication and characterization of metrologically viable epitaxial graphene samples.

  • 5.
    He, Hans
    et al.
    Chalmers University of Technology, Sweden.
    Kim, K. H.
    Danilov, A.
    Montemurro, D.
    Yu, L.
    Park, Y. W.
    Lombardi, F.
    Bauch, T.
    Moth-Poulsen, K.
    Iakimov, T.
    Yakimova, R.
    Malmberg, P.
    MÃŒller, C.
    Kubatkin, S.
    Lara-Avila, S.
    Uniform doping of graphene close to the Dirac point by polymer-assisted assembly of molecular dopants2018Ingår i: Nature Communications, E-ISSN 2041-1723, Vol. 9, nr 1Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Tuning the charge carrier density of two-dimensional (2D) materials by incorporating dopants into the crystal lattice is a challenging task. An attractive alternative is the surface transfer doping by adsorption of molecules on 2D crystals, which can lead to ordered molecular arrays. However, such systems, demonstrated in ultra-high vacuum conditions (UHV), are often unstable in ambient conditions. Here we show that air-stable doping of epitaxial graphene on SiC—achieved by spin-coating deposition of 2,3,5,6-tetrafluoro-tetracyano-quino-dimethane (F4TCNQ) incorporated in poly(methyl-methacrylate)—proceeds via the spontaneous accumulation of dopants at the graphene-polymer interface and by the formation of a charge-transfer complex that yields low-disorder, charge-neutral, large-area graphene with carrier mobilities 70 000 cm2 V−1 s−1 at cryogenic temperatures. The assembly of dopants on 2D materials assisted by a polymer matrix, demonstrated by spin-coating wafer-scale substrates in ambient conditions, opens up a scalable technological route toward expanding the functionality of 2D materials. 

  • 6.
    He, Hans
    et al.
    Chalmers University of Technology, Sweden.
    Lara-Avila, Samuel
    Chalmers University of Technology, Sweden.
    Bergsten, Tobias
    RISE - Research Institutes of Sweden (2017-2019), Säkerhet och transport, Mätteknik.
    Eklund, Gunnar
    RISE - Research Institutes of Sweden (2017-2019), Säkerhet och transport, Mätteknik.
    Kim, Kyung
    Chalmers University of Technology, Sweden.
    Yakimova, Rositza
    Linköping University, Sweden.
    Park, Yung Woo
    Seoul National University, South Korea.
    Kubatkin, Sergey
    Chalmers University of Technology, Sweden.
    Stable and Tunable Charge Carrier Control of Graphene for Quantum Resistance Metrology2018Ingår i: 2018 Conference on Precision Electromagnetic Measurements (CPEM 2018), 2018Konferensbidrag (Refereegranskat)
    Abstract [en]

    Here we demonstrate a stable and tunable method to alter the carrier concentration of epitaxial graphene grown on silicon carbide. This technique relies on chemical doping by an acceptor molecule. Through careful tuning one can produce chemically doped graphene quantum resistance devices which show long-term stability in ambient conditions and have performance comparable to that of GaAs quantum resistance standards. This development paves the way for controlled device fabrication of graphene quantum hall resistance standards, which can be reliably tailored to operate below 5 T and above 4 K out-of-the-box, without further adjustments from the end-user.

  • 7.
    He, Hans
    et al.
    Chalmers University of Technology, Sweden.
    Lara-Avila, Samuel
    Chalmers University of Technology, Sweden; National Physical Laboratory, UK.
    Kim, Kyung
    Chalmers University of Technology, Sweden.
    Fletcher, Nick
    NPL National Physical Laboratory, UK.
    Rozhko, Sergiy
    NPL National Physical Laboratory, UK.
    Bergsten, Tobias
    RISE - Research Institutes of Sweden (2017-2019), Säkerhet och transport, Mätteknik.
    Eklund, Gunnar
    RISE - Research Institutes of Sweden (2017-2019), Säkerhet och transport, Mätteknik.
    Cedergren, Karin
    RISE - Research Institutes of Sweden (2017-2019), Säkerhet och transport, Mätteknik.
    Yakimova, Rositsa
    Linköping University, Sweden.
    Park, Yung
    Seoul National University, South Korea; University of Pennsylvania, US.
    Tzalenchuk, Alexander
    NPL National Physical Laboratory, UK.
    Kubatkin, Sergey
    Chalmers University of Technology, Sweden; University of London, UK.
    Polymer-encapsulated molecular doped epigraphene for quantum resistance metrology2019Ingår i: Metrologia, ISSN 0026-1394, E-ISSN 1681-7575, Vol. 56, nr 4, artikel-id 045004Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    One of the aspirations of quantum metrology is to deliver primary standards directly to end-users thereby significantly shortening the traceability chains and enabling more accurate products. Epitaxial graphene grown on silicon carbide (epigraphene) is known to be a viable candidate for a primary realisation of a quantum Hall resistance standard, surpassing conventional semiconductor two-dimensional electron gases, such as those based on GaAs, in terms of performance at higher temperatures and lower magnetic fields. The bottleneck in the realisation of a turn-key quantum resistance standard requiring minimum user intervention has so far been the need to fine-tune the carrier density in this material to fit the constraints imposed by a simple cryo-magnetic system. Previously demonstrated methods, such as via photo-chemistry or corona discharge, require application prior to every cool-down as well as specialist knowledge and equipment. To this end we perform metrological evaluation of epigraphene with carrier density tuned by a recently reported permanent molecular doping technique. Measurements at two National Metrology Institutes confirm accurate resistance quantisation below 5n-1. Furthermore, samples show no significant drift in carrier concentration and performance on multiple thermal cycles over three years. This development paves the way for dissemination of primary resistance standards based on epigraphene

  • 8.
    He, Hans
    et al.
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik. Chalmers University of Technology, Sweden.
    Shetty, Naveen
    Chalmers University of Technology, Sweden.
    Kubatkin, Sergey
    Chalmers University of Technology, Sweden.
    Stadler, Pascal
    Chalmers University of Technology, Sweden.
    Löfwander, Tomas
    Chalmers University of Technology, Sweden.
    Fogelström, Mikael
    Chalmers University of Technology, Sweden.
    Miranda-Valenzuela, JC
    Tecnologico de Monterrey, Mexico.
    Yakimova, Rositsa
    Linköping University, Sweden; Graphensic AB, Sweden.
    Bauch, Thilo
    Chalmers University of Technology, Sweden.
    Lara-Avila, Samuel
    Chalmers University of Technology, Sweden.
    Highly efficient UV detection in a metal-semiconductor-metal detector with epigraphene2022Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 120, nr 19, artikel-id 191101Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We show that epitaxial graphene on silicon carbide (epigraphene) grown at high temperatures (T >1850 °C) readily acts as material for implementing solar-blind ultraviolet (UV) detectors with outstanding performance. We present centimeter-sized epigraphene metal-semiconductor-metal (MSM) detectors with a peak external quantum efficiency of η ∼85% for wavelengths λ = 250-280 nm, corresponding to nearly 100% internal quantum efficiency when accounting for reflection losses. Zero bias operation is possible in asymmetric devices, with the responsivity to UV remaining as high as R = 134 mA/W, making this a self-powered detector. The low dark currents Io ∼50 fA translate into an estimated record high specific detectivity D = 3.5 × 1015 Jones. The performance that we demonstrate, together with material reproducibility, renders epigraphene technologically attractive to implement high-performance planar MSM devices with a low processing effort, including multi-pixel UV sensor arrays, suitable for a number of practical applications. © 2022 Author(s).

  • 9.
    Janssen, T. J. B. M.
    et al.
    NPL National Physical Laboratory, UK.
    Rozhko, S.
    NPL National Physical Laboratory, UK.
    Antonov, I.
    University of London, UK.
    Tzalenchuk, A.
    NPL National Physical Laboratory, UK; University of London, UK.
    Williams, J. M.
    NPL National Physical Laboratory, UK.
    Melhem, Z.
    He, Hans
    Chalmers University of Technology, Sweden.
    Lara-Avila, S.
    Chalmers University of Technology, Sweden.
    Kubatkin, S.
    Chalmers University of Technology, Sweden.
    Yakimova, R.
    Linköping University, Sweden.
    Operation of graphene quantum Hall resistance standard in a cryogen-free table-top system2015Ingår i: 2D Materials, E-ISSN 2053-1583, Vol. 2, nr 3Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Wedemonstrate quantum Hall resistance measurements with metrological accuracy in a small cryogen-free system operating at a temperature of around 3.8Kand magnetic fields below 5 T. Operating this system requires little experimental knowledge or laboratory infrastructure, thereby greatly advancing the proliferation of primary quantum standards for precision electrical metrology. This significant advance in technology has come about as a result of the unique properties of epitaxial graphene on SiC.

  • 10. Janssen, T. J. B. M.
    et al.
    Rozhko, S.
    Williams, J. M.
    Ireland, J.
    Giblin, S. P.
    He, Hans
    Chalmers University of Technology, Sweden.
    Lara-Avila, S.
    Kubatkin, S.
    Yakimova, R.
    Tzalenchuk, A.
    Towards a cryogen-free table-Top primary resistance standard2016Ingår i: CPEM 2016 - Conference on Precision Electromagnetic Measurements, Conference Digest10 August 2016 Article number 7540654, Institute of Electrical and Electronics Engineers Inc. , 2016Konferensbidrag (Refereegranskat)
    Abstract [en]

    We demonstrate quantum Hall resistance measurements with metrological accuracy in a small cryogen-free system operating at a temperature of around 3.8 K and magnetic fields below 5 T. We use this system to investigate the optimisation of graphene/SiC devices for maximum breakdown current. In addition we report the first characterisation of a cryogen-free cryogenic current comparator which enables entirely cryogen-free primary resistance metrology.

  • 11. Karimi, B.
    et al.
    He, Hans
    Chalmers University of Technology, Sweden.
    Chang, Y. -C
    Wang, L.
    Pekola, J. P.
    Yakimova, R.
    Shetty, N.
    Peltonen, J. T.
    Lara-Avila, S.
    Kubatkin, S.
    Electron-phonon coupling of epigraphene at millikelvin temperatures measured by quantum transport thermometry2021Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 118, nr 10Artikel i tidskrift (Refereegranskat)
  • 12. Kim, K. H.
    et al.
    He, Hans
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik. Chalmers University of Technology, Sweden.
    Rodner, M.
    Yakimova, R.
    Larsson, K.
    Piantek, M.
    Serrate, D.
    Zakharov, A.
    Kubatkin, S.
    Eriksson, J.
    Lara-Avila, S.
    Chemical Sensing with Atomically Thin Platinum Templated by a 2D Insulator2020Ingår i: Advanced Materials Interfaces, ISSN 2196-7350, Vol. 7, nr 12Artikel i tidskrift (Refereegranskat)
    Abstract [en]

  • 13. Kim, K. H.
    et al.
    He, Hans
    Chalmers University of Technology, Sweden.
    Struzzi, C.
    Zakharov, A.
    Giusca, C. E.
    Tzalenchuk, A.
    Park, Y. W.
    Yakimova, R.
    Kubatkin, S.
    Lara-Avila, S.
    Ambipolar charge transport in quasi-free-standing monolayer graphene on SiC obtained by gold intercalation2020Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 102, nr 16Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a study of quasi-free-standing monolayer graphene obtained by intercalation of Au atoms at the interface between the carbon buffer layer (Bu-L) and the silicon-terminated face (0001) of 4H-silicon carbide. Au intercalation is achieved by deposition of an atomically thin Au layer on the Bu-L followed by annealing at 850 °C in an argon atmosphere. We explore the intercalation of Au and decoupling of the Bu-L into quasi-free-standing monolayer graphene by surface science characterization and electron transport in top-gated electronic devices. By gate-dependent magnetotransport we find that the Au-intercalated buffer layer displays all properties of monolayer graphene, namely gate-tunable ambipolar transport across the Dirac point, but we find no observable enhancement of spin-orbit effects in the graphene layer, despite its proximity to the intercalated Au layer. 

  • 14. Kim, K. H.
    et al.
    Lara-Avila, S.
    He, Hans
    Chalmers University of Technology, Sweden.
    Kang, H.
    Hong, S. J.
    Park, M.
    Eklöf, J.
    Moth-Poulsen, K.
    Matsushita, S.
    Akagi, K.
    Kubatkin, S.
    Park, Y. W.
    Probing variable range hopping lengths by magneto conductance in carbonized polymer nanofibers2018Ingår i: Scientific Reports, E-ISSN 2045-2322, Vol. 8, nr 1Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Using magneto transport, we probe hopping length scales in the variable range hopping conduction of carbonized polyacetylene and polyaniline nanofibers. In contrast to pristine polyacetylene nanofibers that show vanishing magneto conductance at large electric fields, carbonized polymer nanofibers display a negative magneto conductance that decreases in magnitude but remains finite with respect to the electric field. We show that this behavior of magneto conductance is an indicator of the electric field and temperature dependence of hopping length in the gradual transition from the thermally activated to the activation-less electric field driven variable range hopping transport. This reveals magneto transport as a useful tool to probe hopping lengths in the non-linear hopping regime. 

  • 15. Kim, K. H.
    et al.
    Lara-Avila, S.
    He, Hans
    Chalmers University of Technology, Sweden.
    Kang, H.
    Park, Y. W.
    Yakimova, R.
    Kubatkin, S.
    Thermal stability of epitaxial graphene electrodes for conductive polymer nanofiber devices2017Ingår i: Crystals, ISSN 2073-4352, Vol. 7, nr 12Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We used large area, monolayer graphene epitaxially grown on SiC (0001) as contact electrodes for polymer nanofiber devices. Our fabrication process, which avoids polymer resist residues on the graphene surface, results in graphene-polyaniline nanofiber devices with Ohmic contacts and electrical conductivity comparable to that of Au-nanofiber devices. We further checked the thermal stability of the graphene contacts to polyaniline devices by annealing up to T = 800 °C, the temperature at which polyaniline nanofibers are carbonized but the graphene electrode remains intact. The thermal stability and Ohmic contact of polymer nanofibers are demonstrated here, which together with the chemical stability and atomic flatness of graphene, make epitaxial graphene on SiC an attractive contact material for future all-carbon electronic devices.

  • 16. Kim, K. H.
    et al.
    Lara-Avila, S.
    Kang, H.
    He, Hans
    Chalmers University of Technology, Sweden.
    Eklöf, J.
    Hong, S. J.
    Park, M.
    Moth-Poulsen, K.
    Matsushita, S.
    Akagi, K.
    Kubatkin, S.
    Park, Y. W.
    Apparent Power Law Scaling of Variable Range Hopping Conduction in Carbonized Polymer Nanofibers2016Ingår i: Scientific Reports, E-ISSN 2045-2322, Vol. 6Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We induce dramatic changes in the structure of conducting polymer nanofibers by carbonization at 800 °C and compare charge transport properties between carbonized and pristine nanofibers. Despite the profound structural differences, both types of systems display power law dependence of current with voltage and temperature, and all measurements can be scaled into a single universal curve. We analyze our experimental data in the framework of variable range hopping and argue that this mechanism can explain transport properties of pristine polymer nanofibers as well. 

  • 17. Lara-Avila, S.
    et al.
    Danilov, A.
    Golubev, D.
    He, Hans
    Chalmers University of Technology, Sweden.
    Kim, K. H.
    Yakimova, R.
    Lombardi, F.
    Bauch, T.
    Cherednichenko, S.
    Kubatkin, S.
    Towards quantum-limited coherent detection of terahertz waves in charge-neutral graphene2019Ingår i: Nature Astronomy, E-ISSN 2397-3366, Vol. 3, nr 11, s. 983-988Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    .

  • 18.
    Mašláň, S.
    et al.
    Czech metrology institute, Czech Republic.
    He, Hans
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik.
    Bergsten, Tobias
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik.
    Seitz, S.
    PTB Physikalisch-Technische Bundesanstalt, Germany.
    Heins, T. P.
    PTB Physikalisch-Technische Bundesanstalt, Germany.
    Interlaboratory Comparison of Low Impedance for Impedance Spectroscopy2022Ingår i: 25th IMEKO TC-4 International Symposium on Measurement of Electrical Quantities, IMEKO TC-4 2022 and 23rd International Workshop on ADC and DAC Modelling and Testing, IWADC 2022Pages 227 - 232, International Measurement Confederation (IMEKO) , 2022, s. 227-232Konferensbidrag (Refereegranskat)
    Abstract [en]

    The paper reports an interlaboratory comparison of low impedance measurements at frequencies relevant for electrochemical impedance spectroscopy (EIS) of commercial lithium-ion cells. The comparisons cover an impedance range from 50 ΌΩ to 100 mΩ across the full complex plane in a frequency range 0.01 Hz up to 5 kHz. A first comparison covered calibration of low impedance standards by reference digital sampling impedance setups in 4-terminal and 4 terminal-pair connections. A second comparison used commercial 4-terminal EIS meters to measure the low impedance standards characterised in the first comparison. 

  • 19.
    Mašláň, S
    et al.
    Czech metrology institute, Czech Republic.
    He, Hans
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik.
    Bergsten, Tobias
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik.
    Seitz, Steffen
    PTB Physikalisch-Technische Bundesanstalt, Germany.
    Heins, Tom P
    PTB Physikalisch-Technische Bundesanstalt, Germany.
    Interlaboratory comparison of battery impedance analyzers calibration2023Ingår i: Measurement, ISSN 0263-2241, E-ISSN 1873-412X, Vol. 218, artikel-id 113176Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The paper reports a results of series of interlaboratory comparisons of low impedance measurements at frequencies relevant for electrochemical impedance spectroscopy (EIS) of commercial lithium-ion cells. Two comparisons are presented. The first, bilateral comparison has focused on low impedance standards calibration in a full complex plane using digital sampling setups. The second comparison has focused on calibration and use of commercial 4-terminal battery EIS meters. Both comparisons have covered the impedance range from 50μ℧ to 100m℧ across the full complex plane in a frequency range from 0.01Hz up to 5kHz. Finally, the paper summarizes practices identified as critical for achieving measurement compatibility among various labs. © 2023 The Authors

  • 20.
    Mašláň, Stanisalav
    et al.
    Czech metrology institute, Czech Republic.
    He, Hans
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik.
    Bergsten, Tobias
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik.
    Seitz, Steffen
    PTB, Germany.
    Heins, Tom P
    PTB, Germany.
    Interlaboratory Comparison of Low Impedance for Impedance Spectroscopy2022Ingår i: 25th IMEKO TC-4 International Symposium on Measurement of Electrical Quantities, IMEKO TC-4 2022 and 23rd International Workshop on ADC and DAC Modelling and Testing, IWADC 2022, International Measurement Confederation (IMEKO) , 2022, s. 227-232Konferensbidrag (Refereegranskat)
    Abstract [en]

    The paper reports an interlaboratory comparison of low impedance measurements at frequencies relevant for electrochemical impedance spectroscopy (EIS) of commercial lithium-ion cells. The comparisons cover an impedance range from 50 μΩ to 100 mΩ across the full complex plane in a frequency range 0.01 Hz up to 5 kHz. A first comparison covered calibration of low impedance standards by reference digital sampling impedance setups in 4-terminal and 4 terminal-pair connections. A second comparison used commercial 4-terminal EIS meters to measure the low impedance standards characterised in the first comparison.

  • 21.
    Shetty, Naveen
    et al.
    Chalmers University of Technology, Sweden.
    Bergsten, Tobias
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik.
    Eklund, Gunnar
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik.
    Avila, Samuel Lara
    Chalmers University of Technology, Sweden; NPL, UK.
    Kubatkin, Sergey
    Chalmers University of Technology, Sweden.
    Cedergren, Karin
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik.
    He, Hans
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik. Chalmers University of Technology, Sweden.
    Long-term stability of molecular doped epigraphene quantum Hall standards: single elements and large arrays (R K/236 ≈ 109 Ω)2023Ingår i: Metrologia, ISSN 0026-1394, E-ISSN 1681-7575, Vol. 60, nr 5, artikel-id 055009Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this work we investigate the long-term stability of epitaxial graphene (epigraphene) quantum Hall resistance standards, including single devices and an array device composed of 236 elements providing (R K/236 ≈ 109 Ω) , with R K the von Klitzing constant. All devices utilize the established technique of chemical doping via molecular dopants to achieve homogenous doping and control over carrier density. However, optimal storage conditions and the long-term stability of molecular dopants for metrological applications have not been widely studied. In this work we aim to identify simple storage techniques that use readily available and cost-effective materials which provide long-term stability for devices without the need for advanced laboratory equipment. The devices are stored in glass bottles with four different environments: ambient, oxygen absorber, silica gel desiccant, and oxygen absorber/desiccant mixture. We have tracked the carrier densities, mobilities, and quantization accuracies of eight different epigraphene quantum Hall chips for over two years. We observe the highest stability (i.e. lowest change in carrier density) for samples stored in oxygen absorber/desiccant mixture, with a relative change in carrier density below 0.01% per day and no discernable degradation of quantization accuracy at the part-per-billion level. This storage technique yields a comparable stability to the currently established best storage method of inert nitrogen atmosphere, but it is much easier to realize in practice. It is possible to further optimize the mixture of oxygen absorber/desiccant for even greater stability performance in the future. We foresee that this technique can allow for simple and stable long-term storage of polymer-encapsulated molecular doped epigraphene quantum Hall standards, removing another barrier for their wide-spread use in practical metrology. 

  • 22.
    Shetty, Naveen
    et al.
    Chalmers University of Technology, Sweden.
    He, Hans
    RISE Research Institutes of Sweden, Säkerhet och transport, Mätteknik. Chalmers University of Technology, Sweden.
    Mitra, Richa
    Chalmers University of Technology, Sweden.
    Huhtasaari, Johanna
    Chalmers University of Technology, Sweden.
    Iordanidou, Konstantina
    Chalmers University of Technology, Sweden.
    Wiktor, Julia
    Chalmers University of Technology, Sweden.
    Kubatkin, Sergey
    Chalmers University of Technology, Sweden.
    Dash, Saroj
    Chalmers University of Technology, Sweden.
    Yakimova, Rositsa
    Linköping University, Sweden.
    Zeng, Lunjie
    Chalmers University of Technology, Sweden.
    Olsson, Eva
    Chalmers University of Technology, Sweden.
    Lara-Avila, Samuel
    Chalmers University of Technology, Sweden; NPL National Physical Laboratory, UK.
    Scalable Fabrication of Edge Contacts to 2D Materials: Implications for Quantum Resistance Metrology and 2D Electronics2023Ingår i: ACS Applied Nano Materials, E-ISSN 2574-0970, Vol. 6, nr 7, s. 6292-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report a reliable and scalable fabrication method for producing electrical contacts to two-dimensional (2D) materials based on the tri-layer resist system. We demonstrate the applicability of this method in devices fabricated on epitaxial graphene on silicon carbide (epigraphene) used as a scalable 2D material platform. For epigraphene, data on nearly 70 contacts result in median values of the one-dimensional (1D) specific contact resistances ρc ∼ 67 Ω·μm and follow the Landauer quantum limit ρc ∼ n-1/2, consistently reaching values ρc < 50 Ω·μm at high carrier densityn. As a proof of concept, we apply the same fabrication method to the transition metal dichalcogenide (TMDC) molybdenum disulfide (MoS2). Our edge contacts enable MoS2 field-effect transistor (FET) behavior with an ON/OFF ratio of >106 at room temperature (>109 at cryogenic temperatures). The fabrication route demonstrated here allows for contact metallization using thermal evaporation and also by sputtering, giving an additional flexibility when designing electrical interfaces, which is key in practical devices and when exploring the electrical properties of emerging materials. © 2023 The Authors. 

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