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In situ TEM studies on the graphitization and growth of nanocrystalline graphene from polymers

Chethala Neelakandhan, Shyam Kumar (2019):
In situ TEM studies on the graphitization and growth of nanocrystalline graphene from polymers.
Darmstadt, Technische Universität, [Online-Edition: https://tuprints.ulb.tu-darmstadt.de/9159],
[Ph.D. Thesis]

Abstract

Graphitization of polymers is an efficient way to synthesize graphenoid (graphene like) materials on different substrates with tunable shape, thickness and properties. [1] This catalyst-free growth results in domain sizes of a few nanometers and has been termed nanocrystalline graphene. Ease of fabrication, better control of shape, thickness and properties comparable to graphene makes ncg an easy to produce alternative for graphene for different technological applications. Since the properties of these graphitized carbon structures are largely affected by the domain size and other defects, a detailed understanding of the graphitization and domain growth as a function of temperature is essential to tailor the properties of the graphitic material. In the present thesis, in situ TEM techniques are employed to understand the graphitization and domain growth of free-standing nanocrystalline graphene thin films prepared by vacuum annealing of a photoresist inside a TEM. HRTEM, selected area electron diffraction (SAED) and electron energy loss spectroscopy (EELS) techniques are used to analyze the graphitization and the evolution of nanocrystalline domains at different temperatures. By in situ heating and current annealing, the present study tries to understand the graphitization and structural changes in the intermediate to ultrahigh temperature range. The in situ studies showed that the graphitization process is highly dynamic in nature with a number of intermediate reactions leading to the formation of different carbon nanostructures. The free-standing membrane showed comparable graphitization to substrate supported films and a two-step growth mechanism was identified. At intermediate temperatures (600-1000 ºC) crystallite growth proceeds by consuming amorphous carbon around the crystallites and at high temperatures (1000- 1200 ºC) growth proceeds by merging of crystallites. The amorphous carbon transforms in two ways, by attaching to the active edges of domains and by catalyst free transformation on the top of graphitic layers. This catalyst free transformation forms new graphitic structures with different size shape and mobility. Some of these carbon nano structures are highly mobile on top of the already graphitized layers, which enabled to study the interaction of these structures with the graphitic substrate at high temperatures. Time resolved HRTEM investigation of the high temperature dynamics of ncg supported by atomistic simulations gave insights into the fundamental processes controlling the graphene growth, high temperature stability/mobility of the carbon nanostructures and their interaction with the graphitic substrate. High temperature in situ HRTEM investigations revealed the formation of graphene nano flakes and cage-like nano structures during graphitization. The study showed that the growth of the domains is mainly by the migration and merging of the graphitic subunits. In addition to lateral merging of domains, experiments also showed a merging of small flakes with an under laying substrate edge, which involves a slow vertical material transfer. In addition to this, strong structural and size fluctuations of individual graphitic subunits at high temperatures were observed. Graphene nano flakes are highly unstable and tend to loose atoms or groups of atoms, while adjacent larger domains grows by the addition of atoms indicating an Ostwald type of ripening occurring in these 2D materials as an additional growth mechanism. Beam off experiments confirmed that the observed dynamics are inherent temperature driven processes and the electron beam only provides additional activation energy increasing the reaction kinetics. Molecular dynamic simulations carried out to estimate the activation energy for the different processes indicates a critical role of defects in the substrate for the observed dynamics. Furthermore in situ current annealing of free-standing ncg constrictions were carried out to understand the dynamics and structural changes at ultrahigh temperatures. Current annealing provides the possibility to reach temperatures in excess of 1200 ºC inside the TEM, which is the maximum temperature possible by commercial MEMS based heating chips. The graphitization at high temperature is comparable to the thermal annealing showing similar crystallite size evolution. However, growth of domains up to 50 nm was observed with current annealing to ultra-high temperatures (T > 2000 ºC). Unlike the formation of well oriented graphite during high temperature annealing, in current annealing of thick samples, formation of large multi walled cage-like structures were observed. The thickness of the sample and the heating rate seems to have a critical influence on the structural evolution during current annealing. These initial observations on comparable graphitization during current annealing at intermediate temperatures, growth of domains, formation of cage-like structures etc., open up new possibilities to tailor the microstructure and conductivity by controlling the thickness and heating rate of the sample.

Item Type: Ph.D. Thesis
Erschienen: 2019
Creators: Chethala Neelakandhan, Shyam Kumar
Title: In situ TEM studies on the graphitization and growth of nanocrystalline graphene from polymers
Language: English
Abstract:

Graphitization of polymers is an efficient way to synthesize graphenoid (graphene like) materials on different substrates with tunable shape, thickness and properties. [1] This catalyst-free growth results in domain sizes of a few nanometers and has been termed nanocrystalline graphene. Ease of fabrication, better control of shape, thickness and properties comparable to graphene makes ncg an easy to produce alternative for graphene for different technological applications. Since the properties of these graphitized carbon structures are largely affected by the domain size and other defects, a detailed understanding of the graphitization and domain growth as a function of temperature is essential to tailor the properties of the graphitic material. In the present thesis, in situ TEM techniques are employed to understand the graphitization and domain growth of free-standing nanocrystalline graphene thin films prepared by vacuum annealing of a photoresist inside a TEM. HRTEM, selected area electron diffraction (SAED) and electron energy loss spectroscopy (EELS) techniques are used to analyze the graphitization and the evolution of nanocrystalline domains at different temperatures. By in situ heating and current annealing, the present study tries to understand the graphitization and structural changes in the intermediate to ultrahigh temperature range. The in situ studies showed that the graphitization process is highly dynamic in nature with a number of intermediate reactions leading to the formation of different carbon nanostructures. The free-standing membrane showed comparable graphitization to substrate supported films and a two-step growth mechanism was identified. At intermediate temperatures (600-1000 ºC) crystallite growth proceeds by consuming amorphous carbon around the crystallites and at high temperatures (1000- 1200 ºC) growth proceeds by merging of crystallites. The amorphous carbon transforms in two ways, by attaching to the active edges of domains and by catalyst free transformation on the top of graphitic layers. This catalyst free transformation forms new graphitic structures with different size shape and mobility. Some of these carbon nano structures are highly mobile on top of the already graphitized layers, which enabled to study the interaction of these structures with the graphitic substrate at high temperatures. Time resolved HRTEM investigation of the high temperature dynamics of ncg supported by atomistic simulations gave insights into the fundamental processes controlling the graphene growth, high temperature stability/mobility of the carbon nanostructures and their interaction with the graphitic substrate. High temperature in situ HRTEM investigations revealed the formation of graphene nano flakes and cage-like nano structures during graphitization. The study showed that the growth of the domains is mainly by the migration and merging of the graphitic subunits. In addition to lateral merging of domains, experiments also showed a merging of small flakes with an under laying substrate edge, which involves a slow vertical material transfer. In addition to this, strong structural and size fluctuations of individual graphitic subunits at high temperatures were observed. Graphene nano flakes are highly unstable and tend to loose atoms or groups of atoms, while adjacent larger domains grows by the addition of atoms indicating an Ostwald type of ripening occurring in these 2D materials as an additional growth mechanism. Beam off experiments confirmed that the observed dynamics are inherent temperature driven processes and the electron beam only provides additional activation energy increasing the reaction kinetics. Molecular dynamic simulations carried out to estimate the activation energy for the different processes indicates a critical role of defects in the substrate for the observed dynamics. Furthermore in situ current annealing of free-standing ncg constrictions were carried out to understand the dynamics and structural changes at ultrahigh temperatures. Current annealing provides the possibility to reach temperatures in excess of 1200 ºC inside the TEM, which is the maximum temperature possible by commercial MEMS based heating chips. The graphitization at high temperature is comparable to the thermal annealing showing similar crystallite size evolution. However, growth of domains up to 50 nm was observed with current annealing to ultra-high temperatures (T > 2000 ºC). Unlike the formation of well oriented graphite during high temperature annealing, in current annealing of thick samples, formation of large multi walled cage-like structures were observed. The thickness of the sample and the heating rate seems to have a critical influence on the structural evolution during current annealing. These initial observations on comparable graphitization during current annealing at intermediate temperatures, growth of domains, formation of cage-like structures etc., open up new possibilities to tailor the microstructure and conductivity by controlling the thickness and heating rate of the sample.

Place of Publication: Darmstadt
Divisions: 11 Department of Materials and Earth Sciences
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences > Material Science > Advanced Electron Microscopy (aem)
Date Deposited: 01 Dec 2019 20:55
Official URL: https://tuprints.ulb.tu-darmstadt.de/9159
URN: urn:nbn:de:tuda-tuprints-91591
Referees: Krupke, Prof. Dr. Ralph and Kübel, Prof. Dr. Christian
Refereed / Verteidigung / mdl. Prüfung: 29 July 2019
Alternative Abstract:
Alternative abstract Language
Die Graphitisierung von Polymeren ist ein effizienter Weg, um graphenähnliche Materialien auf verschiedenen Substraten mit einstellbarer Form, Dicke und Eigenschaften herzustellen. [1] Dieses Wachstum ohne zusätzlichen Katalysator führt zu Domänengrößen von wenigen Nanometern und wird als nanokristallines Graphen bezeichnet. Der einfache Herstellungsprozess, die bessere Kontrolle über Form, Dicke und Eigenschaften, die mit Graphen vergleichbar sind, machen ncg zu einer leicht zu produzierenden Alternative für Graphen im Hinblick auf verschiedene technologische Anwendungen. Da die Eigenschaften dieser graphitisierten Kohlenstoffstrukturen stark von der Domänengröße und Defekten abhängen, ist ein detailliertes Verständnis der Graphitisierung und des Domänenwachstums als Funktion der Temperatur erforderlich, um die Eigenschaften des graphitischen Materials maßzuschneidern. In der vorliegenden Arbeit werden in-situ-TEM-Techniken eingesetzt, um die Graphitisierung und das Domänenwachstum von freistehenden nanokristallinen Graphen-Dünnfilmen zu verstehen, die durch Vakuumglühen eines Photoresists in einem TEM hergestellt wurden. Mithilfe von HRTEM-, SAED- (Selected Area Electron Diffraction) und EELS-Techniken (Electron Energy Loss Spectroscopy) werden die Graphitisierung und die Entwicklung nanokristalliner Domänen bei verschiedenen Temperaturen analysiert. Durch in-situ-Heizen und zusätzlichem Tempern unter Strom soll versucht werden, die Graphitisierung und strukturellen Veränderungen im Mittel- bis Ultrahochtemperaturbereich zu verstehen. Die in-situ-Untersuchungen zeigten, dass der Graphitisierungsprozess von Natur aus sehr dynamisch ist und über eine Reihe von Zwischenreaktionen zur Bildung verschiedener Kohlenstoffnanostrukturen führt. Die freistehende Membran zeigte eine vergleichbare Graphitisierung wie Filme, die von einem Substrat gestützt werden, und ein zweistufiger Wachstumsmechanismus wurde identifiziert. Bei mittleren Temperaturen (600-1000 ºC) setzt sich das Kristallitwachstum durch Verbrauch von amorphem Kohlenstoff, der um die Kristallite liegt, fort. Bei hohen Temperaturen (1000-1200 ºC) erfolgt das Wachstum durch Verschmelzung von Kristalliten. Der amorphe Kohlenstoff wandelt sich auf zwei Arten um, indem er sich einerseits an die aktiven Ränder von Domänen anlagert und andererseits auf graphitischen Schichten katalysatorfrei umgewandelt wird. Diese katalysatorfreie Umwandlung bildet neue graphitische Strukturen mit unterschiedlicher Form und Beweglichkeit. Einige dieser Kohlenstoff-Nanostrukturen, die oberhalb der bereits graphitisierten Schichten liegen, sind hochbeweglich, was es ermöglichte, die Wechselwirkung dieser Strukturen mit dem graphitischen Substrat bei hohen Temperaturen zu untersuchen. Zeitaufgelöste HRTEM-Untersuchungen der Hochtemperaturdynamik von ncg, durch atomistische Simulationen unterstützt, gaben Einblicke in die grundlegenden Prozesse, die das Graphen-Wachstum, die Hochtemperaturstabilität / Mobilität der Kohlenstoffnanostrukturen und ihre Wechselwirkung mit dem graphitischen Substrat steuern. In-situ-HRTEM-Untersuchungen bei hohen Temperaturen ergaben, dass sich während der Graphitisierung Graphen-Nanoflocken und käfigartige Nanostrukturen bilden. Die Studie zeigte, dass das Wachstum der Domänen hauptsächlich durch die Migration und Verschmelzung der graphitischen Untereinheiten erfolgt. Zusätzlich zum lateralen Verschmelzen von Domänen zeigten Experimente auch ein Verschmelzen kleiner Flocken mit einer unterliegenden Substratkante, was einen langsamen vertikalen Materialtransfer mit sich bringt. Darüber hinaus wurden starke Struktur- und Größenschwankungen einzelner graphitischer Untereinheiten bei hohen Temperaturen beobachtet. Graphen-Nano-Flocken sind sehr instabil und neigen dazu, Atome oder Atomgruppen zu verlieren, während benachbarte, größere Domänen durch die Zugabe von Atomen wachsen. Das weist auf einen Ostwald-Reifungstyphin, der in diesen 2D-Materialien als zusätzlichen Wachstumsmechanismus auftritt. Beam-Off-Experimente bestätigten, dass die beobachtete Dynamik inhärente temperaturgetriebene Prozesse sind und der Elektronenstrahl nur zusätzliche Aktivierungsenergie liefert, die die Reaktionskinetik erhöht. Molekulardynamische Simulationen zur Abschätzung der Aktivierungsenergie für die verschiedenen Prozesse weisen auf eine kritische Rolle von Defekten im Substrat für die beobachtete Dynamik hin. Darüber hinaus wurden freistehende ncg-Einschnürungen in-situ-unter Strom getempert, um die Dynamik und strukturellen Veränderungen bei ultrahohen Temperaturen zu verstehen. Das Tempern unter Strom bietet die Möglichkeit, innerhalb des TEMs Temperaturen von deutlich über 1200 ° C zu erreichen. Dieses ist bei kommerziellen MEMS-basierten Heizchips die maximal erreichbare Temperatur. Die Graphitisierung bei hoher Temperatur ist vergleichbar mit dem thermischen Tempern, das eine ähnliche Kristallitgrößenentwicklung zeigt. Beim Tempern unter Strom auf ultrahohe Temperaturen (T> 2000 ºC) wurde jedoch ein Wachstum von Domänen bis zu 50 nm beobachtet. Im Gegensatz zur Bildung von gut orientiertem Graphit während des Hochtemperaturglühens wurde beim gegenwärtigen Glühen von dicken Proben die Bildung von großen mehrwandigen käfigartigen Strukturen beobachtet. Die Dicke der Probe und die Aufheizrate scheinen einen kritischen Einfluss auf die Strukturentwicklung während des Tempern unter Strom zu haben. Diese ersten Beobachtungen zu vergleichbarer Graphitisierung während des Tempern unter Strom bei Zwischentemperaturen, Domänenwachstum, Bildung käfigartiger Strukturen usw. eröffnen neue Möglichkeiten, die Mikrostruktur und Leitfähigkeit durch Steuerung der Dicke und der Aufheizrate der Probe anzupassen.German
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