Browsing by Author "Mackenzie, David M.A."

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  • All-parylene flexible wafer-scale graphene thin film transistor
    (2021-06-15) Kim, Maria; Mackenzie, David M.A.; Kim, Wonjae; Isakov, Kirill; Lipsanen, Harri
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Graphene is an ideal candidate as a component of flexible/wearable electronics due to its two-dimensional nature and low gate bias requirements for high quality devices. However, the proven methods for fabrication of graphene thin film transistors (TFTs) on fixed substrates involve using a sacrificial polymer layer to transfer graphene to a desired surface have led to mixed results for flexible devices. Here, by using the same polymer layer (parylene C) for both graphene transfer and the flexible substrate itself, we produced graphene TFTs on the wafer-scale requiring less than |2 V| gate bias and with high mechanical resilience of 30,000 bending cycles.
  • Case studies of electrical characterisation of graphene by terahertz time-domain spectroscopy
    (2021-02-17) Whelan, Patrick R.; Zhou, Binbin; Bezencenet, Odile; Shivayogimath, Abhay; Mishra, Neeraj; Shen, Qian; Jessen, Bjarke S.; Pasternak, Iwona; Mackenzie, David M.A.; Ji, Jie; Sun, Cunzhi; Seneor, Pierre; Dlubak, Bruno; Luo, Birong; Østerberg, Frederik W.; Huang, Deping; Shi, Haofei; Luo, Da; Wang, Meihui; Ruoff, Rodney S.; Conran, Ben R.; McAleese, Clifford; Huyghebaert, Cedric; Brems, Steven; Booth, Timothy J.; Napal, Ilargi; Strupinski, Wlodek; Petersen, Dirch H.; Forti, Stiven; Coletti, Camilla; Jouvray, Alexandre; Teo, Kenneth B.K.; Centeno, Alba; Zurutuza, Amaia; Legagneux, Pierre; Jepsen, Peter U.; Bøggild, Peter
     A2 Katsausartikkeli tieteellisessä aikakauslehdessä
    Graphene metrology needs to keep up with the fast pace of developments in graphene growth and transfer. Terahertz time-domain spectroscopy (THz-TDS) is a non-contact, fast, and non-destructive characterization technique for mapping the electrical properties of graphene. Here we show several case studies of graphene characterization on a range of different substrates that highlight the versatility of THz-TDS measurements and its relevance for process optimization in graphene production scenarios.
  • Multilayer MoTe2 Field-Effect Transistor at High Temperatures
    (2021-11-23) Ahmed, Faisal; Shafi, Abde Mayeen; Mackenzie, David M.A.; Qureshi, Maaz Ahmed; Fernandez, Henry A.; Yoon, Hoon Hahn; Uddin, Md Gius; Kuittinen, Markku; Sun, Zhipei; Lipsanen, Harri
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Functional 2D material-based devices are likely subjected to high ambient temperatures when integrated into miniaturized circuits for practical applications, which may induce irreversible structural changes in materials and impact the device performance. However, majority of 2D devices' studies focus on room temperature or low-temperature operation conditions. Here, the high-temperature (up to 673 K) electro-thermal response of molybdenum ditelluride (MoTe2) based field-effect transistors is investigated. The optimal annealing temperature of around 500-525 K for the multilayer MoTe2 devices with twofold enhancement in maximum current level, field-effect mobility, and current on-off ratio is identified. Furthermore, MoTe2 devices show the transition of electrical response from gate modulation to the degenerately p-doped (hole dominant) characteristics when the operation temperature increases to approximate to 600 K. The gate-dependent electro-thermal measurements complemented by surface chemistry analysis confirm the near range hopping transport in the MoTe2 channel at high temperature induced by thermally triggered oxidation of MoTe2. These results not only provide the thermal endurance limits of MoTe2 for practical applications, but also indicate the possible applications of MoTe2 for thermal sensing applications.
  • Review of fabrication methods of large-area transparent graphene electrodes for industry
    (2020-06-01) Mustonen, Petri; Mackenzie, David M.A.; Lipsanen, Harri
     A2 Katsausartikkeli tieteellisessä aikakauslehdessä
    Graphene is a two-dimensional material showing excellent properties for utilization in transparent electrodes; it has low sheet resistance, high optical transmission and is flexible. Whereas the most common transparent electrode material, tin-doped indium-oxide (ITO) is brittle, less transparent and expensive, which limit its compatibility in flexible electronics as well as in low-cost devices. Here we review two large-area fabrication methods for graphene based transparent electrodes for industry: liquid exfoliation and low-pressure chemical vapor deposition (CVD). We discuss the basic methodologies behind the technologies with an emphasis on optical and electrical properties of recent results. State-of-the-art methods for liquid exfoliation have as a figure of merit an electrical and optical conductivity ratio of 43.5, slightly over the minimum required for industry of 35, while CVD reaches as high as 419.
  • A universal approach for the synthesis of two-dimensional binary compounds
    (2019-12-01) Shivayogimath, Abhay; Thomsen, Joachim Dahl; Mackenzie, David M.A.; Geisler, Mathias; Stan, Raluca Maria; Holt, Ann Julie; Bianchi, Marco; Crovetto, Andrea; Whelan, Patrick R.; Carvalho, Alexandra; Neto, Antonio H.Castro; Hofmann, Philip; Stenger, Nicolas; Bøggild, Peter; Booth, Timothy J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Only a few of the vast range of potential two-dimensional materials (2D) have been isolated or synthesised to date. Typically, 2D materials are discovered by mechanically exfoliating naturally occurring bulk crystals to produce atomically thin layers, after which a material-specific vapour synthesis method must be developed to grow interesting candidates in a scalable manner. Here we show a general approach for synthesising thin layers of two-dimensional binary compounds. We apply the method to obtain high quality, epitaxial MoS2 films, and extend the principle to the synthesis of a wide range of other materials—both well-known and never-before isolated—including transition metal sulphides, selenides, tellurides, and nitrides. This approach greatly simplifies the synthesis of currently known materials, and provides a general framework for synthesising both predicted and unexpected new 2D compounds.
  • Wafer-scale graphene quality assessment using micro four-point probe mapping
    (2020-05-29) Mackenzie, David M.A.; Kalhauge, Kristoffer G.; Whelan, Patrick R.; Østergaard, Frederik W.; Pasternak, Iwona; Strupinski, Wlodek; Bøggild, Peter; Jepsen, Peter U.; Petersen, Dirch H.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Micro four-point probes (M4PP) provide rapid and automated lithography-free transport properties of planar surfaces including two-dimensional materials. We perform sheet conductance wafer maps of graphene directly grown on a 100 mm diameter SiC wafer using a multiplexed seven-point probe with minor additional measurement time compared to a four-point probe. Comparing the results of three subprobes we find that compared to a single-probe result, our measurement yield increases from 72%-84% to 97%. The additional data allows for correlation analysis between adjacent subprobes, that must measure the same values in case the sample is uniform on the scale of the electrode pitch. We observe that the relative difference in measured sheet conductance between two adjacent subprobes increase in the transition between large and low conductance regions. We mapped sheet conductance of graphene as it changed over several weeks. Terahertz time-domain spectroscopy conductivity maps both before and after M4PP mapping showed no significant change due to M4PP measurement, with both methods showing the same qualitative changes over time.