Reactive plasma cleaning and restoration of transition metal dichalcogenide monolayers

The cleaning of two-dimensional (2D) materials is an essential step in the fabrication of future devices, leveraging their unique physical, optical, and chemical properties. Part of these emerging 2D materials are transition metal dichalcogenides (TMDs). So far there is limited understanding of the cleaning of “monolayer” TMD materials. In this study, we report on the use of downstream H2 plasma to clean the surface of monolayer WS2 grown by MOCVD. We demonstrate that high-temperature processing is essential, allowing to maximize the removal rate of polymers and to mitigate damage caused to the WS2 in the form of sulfur vacancies. We show that low temperature in situ carbonyl sulfide (OCS) soak is an efficient way to resulfurize the material, besides high-temperature H2S annealing. The cleaning processes and mechanisms elucidated in this work are tested on back-gated field-effect transistors, confirming that transport properties of WS2 devices can be maintained by the combination of H2 plasma cleaning and OCS restoration. The low-damage plasma cleaning based on H2 and OCS is very reproducible, fast (completed in a few minutes) and uses a 300 mm industrial plasma etch system qualified for standard semiconductor pilot production. This process is, therefore, expected to enable the industrial scale-up of 2D-based devices, co-integrated with silicon technology

HED-TIE: A wafer-scale approach for fabricating hybrid electronic devices with trench isolated electrodes and its application in sensing devices

Die organisch-anorganische Hybridelektronik bietet verschiedene Möglichkeiten zur Entwicklung neuartiger Bauelemente, welche die Vorteile von organischen und anorganischen Halbleitern vereinen. Planare Bauelemente werden typischerweise mittels Schattenmasken-basierter Strukturierung hergestellt. Ein Grund hierfür ist die Empfindlichkeit organischer Halbleiter gegenüber Ultraviolettem Licht und Lösungsmitteln, welche in den Standard-Photolithographieprozessen eingesetzt werden. Die Schattenmasken-Strukturierung führt allerdings zu Bauelementen mit kleinsten Abmessungen im Mikrometerbereich. Für die Reduzierung der Kanalabmessungen von planaren organisch-anorganischen Hybridbauelementen unterhalb eines Mikrometers ist die Elektronenstrahllithographie die am häufigsten verwendete Technik. Aufgrund des hohen Kosten- und Zeitaufwandes ist es nicht möglich, diese Technik für Wafermaßstab-Herstellung in der industriellen Anwendung einzusetzen. In dieser Arbeit wird eine alternative Technologie zur Herstellung von planaren Bauelementen mit isolierten Grabenelektroden und Kanalabmessungen von wenigen Hundert Nanometer bis unter 100 nm vorgestellt. Gräben kleiner als ein Mirkometer werden zunächst auf Silizium-Substraten strukturiert und anschließend mit einer isolierenden SiO2 Schicht aufgefüllt. Diese hilft dabei die gewünschten Elektrodenabstände, also die gewünschte Kanallänge, zu erreichen. Die Flexibilität des neuen Herstellungsverfahrens ermöglicht es nicht nur verschiedenen Kanallängen und Bauelement-Geometrie, sondern auch die Verwendung verschiedener Materialien für Elektroden und organischen Kanäle. Dies wiederum ermöglicht eine Vielfalt von potentiellen Anwendungen der hybriden Bauelemente. In dieser Arbeit wurde 6,13-bis (triisopropylsilylethinyl)-Pentacen (TIPS-Pentacen) Lösung und metallfreie Phthalocyanin als organisches Material verwendet und als Elektrodenmaterial diente Gold. Die entstandenen auf TIPS-Pentacen-Lösung basierenden planaren hybriden Bauelemente wurden für potentielle Anwendungen als optische sowie magnetoresistive Sensoren getestet.:Table of Contents Bibliografische Beschreibung 1 Chapter 1. Introduction 3 1.1 Organic-inorganic hybrid electronics 4 1.2 Inorganic semiconductors versus organic semiconductors 5 1.3 Electronic properties of a molecular layer 5 1.4 Vertical HEDs and planar HEDs 6 Chapter 2. Wafer-scale fabrication approach for planar HED-TIEs 8 2.1 Overview of nano-patterning techniques 8 (a) Electron beam lithography (EBL) 8 (b) Nanostencil lithography (NSL) 8 (c) Nanoimprint lithography (NIL) 9 2.2 Fabrication of planar organic-inorganic HED-TIEs 12 2.2.1 Trench refill approach for fabricating HED-TIEs 12 2.2.1.1 Deposition of the trench refill layer 15 2.2.1.2 Deposition of the organic channel material 16 (a) HED-TIE with thermally evaporated organic channel 16 (b) HED-TIE with solution processed organic channel 18 2.2.2 Spacer approach for fabricating HED-TIEs 21 2.2.2.1 Deposition of the isolation layer 23 2.3 Characterization techniques 26 (a) Electrical characterization 26 (b) Raman spectroscopy 26 (c) Photoluminescence spectroscopy 27 2.4 Summary and outlook 27 Chapter 3. Electrical characterization of HED-TIEs 29 3.1 Theoretical background 29 3.1.1 Space charge limited current (SCLC) conduction mechanism 29 3.2 Experimental details 32 3.3 Results and discussions 34 3.4 Summary and outlook 40 Chapter 4. Application of HED-TIEs as optical sensors 41 4.1 Photosensing properties of TIPS-pentacene based HED-TIEs 41 4.1.1 Theoretical background 41 4.1.2 Experimental details 43 4.1.3 Results and discussions 44 4.1.4 Summary and outlook 49 4.2 Photosensing properties of TIPS-pentacene based HED-TIEs with Au nanoparticles in the channel matrix 50 4.2.1 Theoretical background 50 4.2.2 Experimental details 51 4.2.3 Results and discussions 52 4.2.4 Summary and outlook 59 Chapter 5. Application of HED-TIE devices as magnetoresistive sensors 61 5.1 Theoretical background 61 5.1.1 Organic spintronics 61 5.1.2 Mechanisms of organic magnetoresistance (OMAR) 65 (a) Bipolaron model 68 (b) Electron-hole (e-h) pair model 69 (c) Exciton–charge interaction model 70 5.2. OMAR measurements on TIPS-pentacene OFETs and HED-TIEs 71 5.2.1 Experimental details 71 5.3 Results and discussions 73 5.4 Summary and outlook 79 Chapter 6. Summary and outlook 81 References 86 List of Figures 97 List of Tables 103 List of Abbreviations 104 Acknowledgements 106 List of Publications 108 List of Conference Presentations and Posters 109 Selbstständigkeitserklärung 111 Curriculum Vitae 11

Light-induced magnetoresistance in solution-processed planar hybrid devices measured under ambient conditions

We report light-induced negative organic magnetoresistance (OMAR) measured in ambient atmosphere in solution-processed 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) planar hybrid devices with two different device architectures. Hybrid electronic devices with trench-isolated electrodes (HED-TIE) having a channel length of ca. 100 nm fabricated in this work and, for comparison, commercially available pre-structured organic field-effect transistor (OFET) substrates with a channel length of 20 µm were used. The magnitude of the photocurrent as well as the magnetoresistance was found to be higher for the HED-TIE devices because of the much smaller channel length of these devices compared to the OFETs. We attribute the observed light-induced negative magnetoresistance in TIPS-pentacene to the presence of electron–hole pairs under illumination as the magnetoresistive effect scales with the photocurrent. The magnetoresistance effect was found to diminish over time under ambient conditions compared to a freshly prepared sample. We propose that the much faster degradation of the magnetoresistance effect as compared to the photocurrent was due to the incorporation of water molecules in the TIPS-pentacene film

Nanoantenna-assisted plasmonic enhancement of IR absorption of vibrational modes of organic molecules

Nanoantenna-assisted plasmonic enhancement of IR absorption and Raman scattering was employed for studying the vibrational modes in organic molecules. Ultrathin cobalt phthalocyanine films (3 nm) were deposited on Au nanoantenna arrays with specified structural parameters. The deposited organic films reveal the enhancement of both Raman scattering and IR absorption vibrational modes. To extend the possibility of implementing surface-enhanced infrared absorption (SEIRA) for biological applications, the detection and analysis of the steroid hormone cortisol was demonstrated

Reactive plasma cleaning and restoration of transition metal dichalcogenide monolayers

The cleaning of two-dimensional (2D) materials is an essential step in the fabrication of future devices, leveraging their unique physical, optical, and chemical properties. Part of these emerging 2D materials are transition metal dichalcogenides (TMDs). So far there is limited understanding of the cleaning of “monolayer” TMD materials. In this study, we report on the use of downstream H2 plasma to clean the surface of monolayer WS2 grown by MOCVD. We demonstrate that high-temperature processing is essential, allowing to maximize the removal rate of polymers and to mitigate damage caused to the WS2 in the form of sulfur vacancies. We show that low temperature in situ carbonyl sulfide (OCS) soak is an efficient way to resulfurize the material, besides high-temperature H2S annealing. The cleaning processes and mechanisms elucidated in this work are tested on back-gated field-effect transistors, confirming that transport properties of WS2 devices can be maintained by the combination of H2 plasma cleaning and OCS restoration. The low-damage plasma cleaning based on H2 and OCS is very reproducible, fast (completed in a few minutes) and uses a 300 mm industrial plasma etch system qualified for standard semiconductor pilot production. This process is, therefore, expected to enable the industrial scale-up of 2D-based devices, co-integrated with silicon technology

Nucleation and coalescence of tungsten disulfide layers grown by metalorganic chemical vapor deposition

International audienc

Apoptotic Mode of Cell Death in Substantia Nigra Following Intranigral Infusion of the Parkinsonian Neurotoxin, MPP+ in Sprague-Dawley Rats: Cellular, Molecular and Ultrastructural Evidences

The potent parkinsonian neurotoxin 1-methyl- 4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) is known to cause dopaminergic neurodegeneration in nigrostriatal system. In the present study we investigated the nuclear morphology of cells in the substantia nigra pars compacta (SNpc) region of rats following unilateral intranigral infusion of the active metabolite, 1-methyl-4-phenyl pyridinium ion (MPP+), which resulted in a dose-dependent and prolonged dopamine depletion in the ipsilateral striatum. There appeared a substantial loss of tyrosine hydroxylase immunoreactive neurons in the SNpc that received the neurotoxin. Specific nuclear staining with Hoechst 33342 or acridine orange revealed bright pyknotic, shrunken, distorted nuclei and condensed chromatin with perinuclear nucleolus respectively following visualization with the former and latter dyes in the ipsilateral SNpc, as compared to the round, intact nuclei and centrally positioned nucleolus in the contralateral side. Ultrastructural details of the nucleus under transmission electron microscope confirmed distorted nuclear organization with shrunken or condensed nuclei and disrupted nuclear membrane. These features are typical of nucleus undergoing apoptosis, and suggest that MPP+ causes dopaminergic neuronal death through an apoptotic mode. Typical laddering pattern of genomic DNA isolated from the ipsilateral SN in agarose gel electrophoresis conclusively established apoptosis following intranigral administration of MPP+ in rats