Single indium atoms and few-atom indium clusters anchored onto graphene via silicon heteroatoms

Single atoms and few-atom nanoclusters are of high interest in catalysis and plasmonics, but pathways for their fabrication and stable placement remain scarce. We report here the self-assembly of room-temperature-stable single indium (In) atoms and few-atom In clusters (2-6 atoms) that are anchored to substitutional silicon (Si) impurity atoms in suspended monolayer graphene membranes. Using atomically resolved scanning transmission electron microscopy (STEM), we find that the exact atomic arrangements of the In atoms depend strongly on the original coordination of the Si anchors in the graphene lattice: Single In atoms and In clusters with 3-fold symmetry readily form on 3-fold coordinated Si atoms, whereas 4-fold symmetric clusters are found attached to 4-fold coordinated Si atoms. All structures are produced by our fabrication route without the requirement for electron-beam induced materials modification. In turn, when activated by electron beam irradiation in the STEM, we observe in situ the formation, restructuring and translation dynamics of the Si-anchored In structures: Hexagon-centered 4-fold symmetric In clusters can (reversibly) transform into In chains or In dimers, whereas C-centered 3-fold symmetric In clusters can move along the zig-zag direction of the graphene lattice due to the migration of Si atoms during electron-beam irradiation, or transform to Si-anchored single In atoms. Our results provide a novel framework for the controlled self-assembly and heteroatomic anchoring of single atoms and few-atom clusters on graphene

Direct observation of layer-stacking and oriented wrinkles in multilayer hexagonal boron nitride

Hexagonal boron nitride (h-BN) has long been recognized as an ideal substrate for electronic devices due to its dangling-bond-free surface, insulating nature and thermal/chemical stability. Therefore, to analyse the lattice structure and orientation of h-BN crystals becomes important. Here, the stacking order and wrinkles of h-BN are investigated by transmission electron microscopy (TEM). It is experimentally confirmed that the layers in the h-BN flakes are arranged in the AA' stacking. The wrinkles in a form of threefold network throughout the h-BN crystal are oriented along the armchair direction, and their formation mechanism was further explored by molecular dynamics simulations. Our findings provide a deep insight about the microstructure of h-BN and shed light on the structural design/electronic modulations of two-dimensional crystals.Comment: 7 pages, 5 figure

Isolating hydrogen in hexagonal boron nitride bubbles by a plasma treatment

Atomically thin hexagonal boron nitride (h-BN) is often regarded as an elastic film that is impermeable to gases. The high stabilities in thermal and chemical properties allow h-BN to serve as a gas barrier under extreme conditions.In this work, we demonstrate the isolation of hydrogen in bubbles of h-BN via plasma treatment.Detailed characterizations reveal that the substrates do not show chemical change after treatment. The bubbles are found to withstand thermal treatment in air,even at 800 degree celsius. Scanning transmission electron microscopy investigation shows that the h-BN multilayer has a unique aligned porous stacking nature, which is essential for the character of being transparent to atomic hydrogen but impermeable to hydrogen molecules. We successfully demonstrated the extraction of hydrogen gases from gaseous compounds or mixtures containing hydrogen element. The successful production of hydrogen bubbles on h-BN flakes has potential for further application in nano/micro-electromechanical systems and hydrogen storage.Comment: 55 pages, 33figure

Aerosol Jet Printing of Graphene and Carbon Nanotube Patterns on Realistically Rugged Substrates

Direct-write additive manufacturing of graphene and carbon nanotube (CNT) patterns by aerosol jet printing (AJP) is promising for the creation of thermal and electrical interconnects in (opto)electronics. In realistic application scenarios, this however often requires deposition of graphene and CNT patterns on rugged substrates such as, for example, roughly machined and surface oxidized metal block heat sinks. Most AJP of graphene/CNT patterns has thus far however concentrated on flat wafer-or foil type substrates. Here, we demonstrate AJP of graphene and single walled CNT (SWCNT) patterns on realistically rugged plasma electrolytic-oxidized (PEO) Al blocks, which are promising heat sink materials. We show that AJP on the rugged substrates offers line resolution of down to similar to 40 mu m width for single AJP passes, however, at the cost of noncomplete substrate coverage including noncovered mu m-sized pores in the PEO Al blocks. With multiple AJP passes, full coverage including coverage of the pores is, however, readily achieved. Comparing archetypical aqueous and organic graphene and SWCNT inks, we show that the choice of the ink system drastically influences the nanocarbon AJP parameter window, deposit microstructure including crystalline quality, compactness of deposit, and inter/intrapass layer adhesion for multiple passes. Simple electrical characterization indicates aqueous graphene inks as the most promising choice for AJP-deposited electrical interconnect applications. Our parameter space screening thereby forms a framework for rational process development for graphene and SWCNT AJP on application-relevant, rugged substrates

Investigation of magnetoconductivity and surface properties of exfoliated and epitaxially grown graphene samples

Bu çalışmada, grafit soyma ve epitaksiyel yöntemlerle elde edilmiş grafen numunelerinin yüzey özellikleri ve manyetoiletim özellikleri incelendi. Grafen numunelerinin yüzey özellikleri, optik mikroskop, AFM ve SEM kullanılarak incelendi. Epitaksiyel grafen numunelerinin tabaka sayısını belirlemek için Raman spektrumu ölçüldü. Hall etkisi ve I-V ölçümleri için optik ve elektron-demeti litografi yöntemleri kullanılarak grafen aygıtlar üretildi. Grafit soyma ve epitaksiyel yöntemlerle elde edilmiş grafen numunelerinin I-V karakteristikleri ölçüldü. Işığın, grafenin manyetoiletim özellikleri üzerine etkisini incelemek için, sabit manyetik alan altında, karanlık ve daha sonra aydınlık koşullarda, 25?300 K sıcaklık aralığında Hall etkisi ölçümleri yapıldı. Helyum gazının ve suyun epitaksiyel grafenin Hall ölçümleri üzerine etkilerini incelemek için sabit manyetik alan altında, 30?300 K sıcaklık aralığında Hall etkisi ölçümleri yapıldı. Grafenle helyum gazının etkileşmesini daha iyi anlamak için DFT hesaplamaları yapıldı.In this study, the properties of the surface and magnetoconductivity of exfoliated and epitaxially grown graphene samples were investigated. The properties of the surface of graphene samples were investigated by using optical microscope, AFM, and SEM. The Raman spectrum was measured to determine the number of layers which the epitaxial graphene samples have. The graphene devices were fabricated by using optical and e-beam lithography methods for Hall effect and I-V measurements. I-V characteristics of the exfoliated and epitaxially grown graphene samples were measured. To investigate the effect of light on the properties of magnetoconductivity of graphene, under of a fixed magnetic field, Hall effect measurements were carried out under dark and then illumination conditions at the 25-300 K temperature range. To investigate the effects of helium gas and water on Hall measurements of epitaxial graphene, under of a fixed magnetic field, Hall effect measurements were carried out at the 30-300 K temperature range. To better understand the interaction of helium with graphene, DFT calculations were performed

Defects and deformations in two-dimensional materials analyzed by electron and scanning probe microscopy

Die Eigenschaften zweidimensionaler Materialien lassen sich durch kontrollierte Verformung beeinflussen. In dieser Dissertation werden daher Defekte und Deformationen untersucht die Verformungen verursachen können. Der Inhalt dieser Arbeit ist in zwei Teile gegliedert: Im ersten Teil werden Defekte in zweidimensionalen Materialien mittels hochauflösender Transmissionselektronenmikroskopie untersucht. Der Fokus im zweiten Teil liegt auf Deformationsexperimenten und deren Analyse mittels Ramanspektroskopie und Rastersondenmikroskopie (RSM). Zunächst werden Korngrenzen in durch chemische Gasphasenabscheidung gewachsenem Molybdändisulfid mittels hochaufgelöster Transmissionselektronenmikroskopie untersucht. Dabei stellt sich heraus, dass diese aus Versetzungskernen bestehen die wiederum Spannungen verursachen und zur Ausbildung von Nanoporen führen, deren Größe von Misorientierungswinkel zwischen den Körnern abhängt. Als nächstes zeigen wir neue Arten von intrinsischen und elektronenstrahlinduzierten Punktdefekten un Molybdänditellurid. Da Molybdänditellurid unter Bestrahlung mit Elektronen instabil ist haben wir intrinsische Defekte in diesem Material in Graphenverkapselten Proben untersucht. Zusätzlich zu strukturellen Phasenumwandlungen (z.B. hexagonal -> oktaedrisch -> verzerrt oktaedrisch) wurden unter Bestrahlung mit Elektronen auch neuartige Punktdefekte, wie zum Beispiel dreizählig rotationssymmetrische Quantenpunkte und achsensymmetrische sowie zweizählig rotationssymmetrische Quantendrähte, gefunden. Weiterhin zeigen wir, dass der Rand von verkapseltem Molybdänditellurid unter Elektronenbestrahlung einen ungeordneten, flüssigkeitsartigen Zustand einnimmt. Im zweiten Teil werden die durch eine Rastersonde verursachten Deformationen mittels kombinierter Rasterkraftmikroskopie (RKM)/Ramanspektroskie und Doppelspitzen RSM untersucht. Als Erstes zeigen wir mit Hilfe von gleichzeitigen RKM- und Ramanspektroskopiemessungen, wie der Kontakt zwischen einer nm-großen Rastersondenspitze und einer mehrlagigen-Graphenmembran in selbiger zu einem zweidimensionalen Spannungsfeld auf µm-Skala führt. Zuletzt präsentieren wir spitzeninduzierte, punktartige Verformungen die mit einer zweiten, unabhängen Rastersonde im Doppelspitzen-RSM vermessen wurden. Zusätzlich zu den Verformungsmessungen führen wir zwei neuartige Abbildungstechniken ein, die dazu dienen Spitzen-Proben Wechselwirkungen und stark lokalisierte Signale von einer Punktquelle darzustellen. Eine der Methoden, nämlich die elektrische Übersprechabbildung, ist vielversorechend für zukünftige Transportmessungen auf atomarer Skala. Die Ergebnisse die wir in dieser Dissertation präsentieren zeigen, dass die gezielte Beeinflussung von Defekten und Verformungen in zweidimensionalen Materialen durch Elektronenbestrahlung, CVD Wachstum, Verkapselung und Indentierung es zulässt, ihre elektronischen und morhphologischen Eigenschaften zu verbessern. Dadurch können neue, funktionale Materialien mit den erwünschten Eingenschaften für zukünftige technologische Anwendungen erhalten werden.The properties of two dimensional materials can be tailored by controlling the strain. Defects and deformations, introducing strain in materials, are analyzed by high resolution electron and scanning probe microscopy techniques in this dissertation. The content of the dissertation is divided in two parts. The first part involves high resolution transmission electron microscopy analysis of defects in two dimensional materials. The second part presents deformation experiments in two dimensional materials analyzed through Raman spectroscopy and scanning probe microscopy techniques. First, grain boundaries in molybdenum disulfide grown by chemical vapor deposition is probed by high resolution scanning transmission electron microscopy. We find that they consist of dislocation cores that produce strain and lead to the formation of nanopores with different sizes varying with the misorientation angle between the grains. Next, we reveal new types of intrinsic and electron-beam-induced point defects in molybdenum ditelluride. Because of its instability under electron irradiation, we also analyze graphene encapsulated molybdenum ditelluride to explore its intrinsic defects. In addition to structural phase transitions (e.g. hexagonal -> octahedral -> distorted octahedral) under electron irradiation, the point defects, which have not been observed in this material before, three-fold rotationally symmetric quantum dots and both reflection and two fold rotational symmetric quantum wires for example, are also shown. Further, we present how the edge of encapsulated molybdenum ditelluride shows a disordered liquid-like nature under electron irradiation. In the second part, the large scale local deformations induced by a scanning probe tip are analyzed using the combined AFM/Raman spectroscopy and double-tip SPM techniques. First, we show how the contact of nm-sized scanning probe tip results in a two-dimensional strain field with µm dimensions in the free-standing few-layer graphene membrane through simultaneous AFM and Raman spectroscopy measurements. Finally we present tip-induced point-like deformations measured by another independent scanning probe tip using double-tip SPM. In addition to deformation measurements, we introduce two new imaging techniques to reveal tip-sample interactions and visualize highly localized electrical signal received from a point source. One of the methods, electrical cross-talk imaging, show great promise for measuring the transport over one atom scale in the future. The findings, that we present in this dissertation, show that engineering the defects and deformations in 2D materials through electron-beam irradiation, CVD growth, encapsulation or indentation may enable tuning their electronic and morphological properties. Hence new functional materials with desired physical properties can be obtained for future technological applications