A Review on Mechanics and Mechanical Properties of 2D Materials - Graphene and Beyond

Since the first successful synthesis of graphene just over a decade ago, a variety of two-dimensional (2D) materials (e.g., transition metal-dichalcogenides, hexagonal boron-nitride, etc.) have been discovered. Among the many unique and attractive properties of 2D materials, mechanical properties play important roles in manufacturing, integration and performance for their potential applications. Mechanics is indispensable in the study of mechanical properties, both experimentally and theoretically. The coupling between the mechanical and other physical properties (thermal, electronic, optical) is also of great interest in exploring novel applications, where mechanics has to be combined with condensed matter physics to establish a scalable theoretical framework. Moreover, mechanical interactions between 2D materials and various substrate materials are essential for integrated device applications of 2D materials, for which the mechanics of interfaces (adhesion and friction) has to be developed for the 2D materials. Here we review recent theoretical and experimental works related to mechanics and mechanical properties of 2D materials. While graphene is the most studied 2D material to date, we expect continual growth of interest in the mechanics of other 2D materials beyond graphene

Dry-transfer of chemical vapour deposited nanocarbon thin films

This thesis presents the development of chemical vapour deposited (CVD) graphene and multi-walled carbon nanotubes (MWCNTs) as enabling technologies for flexible transparent conductors offering enhanced functionality. The technologies developed could be employed as thin film field emission sources, optical sensors and substrate-free wideband optical polarisers. Detailed studies were performed on CVD Fe and Ni catalysed carbon nanotubes and nanofibres on indium tin oxide, aluminium and alumina diffusion barriers. Activations energies of 0.5 and 1.5 eV were extracted supporting surface diffusion limited catalysis for CNTs and CNFs. For the first time an activation energy of 2.4 eV has been determined for Cu-catalysed growth of CVD graphene. Graphene was shown to deviate significantly from the more traditional rate-limited surface diffusion and suggests carbon-atom-latticeintegration limited catalysis. An aligned dry-transferred MWCNT thin film fabrication technique was developed using MWCNTs of varied lengths to control the optical transparency and conductivity. A process based on the hot-press lamination of bilayer CVD graphene (HPLG) was also developed. Transport studies revealed that these thin films behave, in a macroscopic sense, similar to traditional c-axis conductive graphite and deviate toward tunnel dominated conduction with increasing degrees of network disorder. Various MWCNT-based thin film field emitters were considered. Solution processing was shown to augment the surface work function of the MWCNTs resulting in reduced turn-on electric fields. Integrated zinc oxide nanowires were investigated and were shown to ballast the emission, thereby preventing tip burn out, and offered lower than expected turn-on fields due to the excitation of a hot electron population. To obviate nearest neighbour electrostatic shielding effects an electrochemical catalyst activation procedure was developed to directly deposit highly aligned sparse carbon nanofibres on stainless steel mesh. Highly-aligned free-standing MWCNT membranes were fabricated through a solid-state peeling technique. Membranes were spanned across large distances thereby offering an ideal platform to investigate the unambiguous photoresponse of MWCNTs by removing all extraneous substrate interfaces, charge traps and nanotube-electrode Shottky barriers as well as using pure, chemically untreated material. Oxygen physisorbtion was repeatedly implicated through in-situ lasing and in-situ heated EDX measurements, FT-IR and lowtemperature transport and transfer measurements. A MWCNT membrane absorptive polariser was fabricated. Polarisers showed wideband operation from 400 nm to 1.1 μm and offered operation over greater spectral windows than commercially available polymer and glass-support dichroic films. Ab-initio simulations showed excellent agreement with the measured polarisation attributing the effect to long-axis selective absorption

Electro-mechanical Reliability of CNT-based Conductive Films on Flexible Substrates

Leitfähige Bahnen oder Schichten auf flexiblen Substraten sind wesentliche Komponenten für die flexible Elektronik. Sie müssen bei äußeren Verformungen ihre elektrische Leitfähigkeit und mechanische Integrität erhalten. Leiterbahnen aus herkömmlichen Materialien wie Indiumzinnoxid (ITO) und Metallen haben auf flexiblen Substraten unter großen Dehnungen nur eine begrenzte Zuverlässigkeit gezeigt. In diesem Zusammenhang könnten Kohlenstoffnanoröhren (CNTs) aufgrund ihrer hervorragenden mechanischen, elektrischen und thermischen Eigenschaften eine vielversprechende Alternative sein. Einige Studien haben berichtet, dass CNT-basierte Leiterbahnen auf flexiblen Substraten sehr großen Zugspannungen standhalten können. Es wurden jedoch nur sehr wenige systematische Studien zu ihrer elektromechanischen Zuverlässigkeit durchgeführt. Diese Doktorarbeit befasst sich mit dem elektromechanischen Verhalten von CNT-basierten Filmen bei statischen und zyklischen Verformungen und untersucht systematisch, die zugrundeliegenden Mechanismen für ihre hervorragende Zuverlässigkeit. Diese Erkenntnisse könnten für die Optimierung potenzieller flexibler Geräte unter Verwendung von CNTs wichtig sein. CNT-basierte Leiterbahnen können durch Tintenstrahldruck oder Schleuderbeschichtung hergestellt werden. Als eines der Standardsubstrate für die gedruckte Elektronik wird eine Polyethylenterephthalat (PET)-Folie als Substrat verwendet. CNT-basierte Leiterbahnen konnten erfolgreich gedruckt werden. Die elektrische Leitfähigkeit ist jedoch durch den geringeren Gehalt an CNTs in der Tinte begrenzt. Daher wurde zur Erzielung einer besseren elektrischen Leitfähigkeit eine Schleuderbeschichtung von CNT-Dispersionen angewendet. Durch eine mehrschichtige Abscheidung wurde ein Schichtwiderstand von lediglich 400 Ohm/sq. erreicht. Der wichtigste Teil dieser Doktorarbeit sind die statischen und zyklischen elektromechanischen Prüfungen sowie die Mikrostrukturcharakterisierung von CNT-basierten leitfähigen Filmen auf flexiblen Substraten. Es wurden Mikrozugversuche durchgeführt, um ihre Zuverlässigkeit unter statischen Zugspannungen zu testen, während Biegeermüdungsversuche durchgeführt wurden, um ihre Lebensdauer unter zyklischen Zugdehnungen zu bewerten. Um das elektromechanische Verhalten von CNT leitfähige Filmen unter Verformung zu untersuchen, wurden ihre Morphologie und änderungen der Mikrostruktur untersucht, indem verschiedene mikroskopische Charakterisierungsmethoden kombiniert wurden. CNT leitfähige Schichte zeigen eine ausgezeichnete Zuverlässigkeit bei hohen Dehnungen von bis zu 50%. Ihre Widerstands-Dehnungs-Abhängigkeit zeigt, dass ihre intrinsischen Leitfähigkeiten durch Strecken sogar verbessert werden. Ihre Dehnbarkeit von bis zu 50% wird durch die Überbrückungswirkung von CNTs über eventuell auftretende lokale Risse begünstigt. Biegeermüdungstests ergaben ferner sehr hohe Lebensdauern der CNT Schichten. Bei kleinen Dehnungsamplituden, von 1% bis 2%, sind die CNT leitfähigen Schichten bis zu 1 Million Biegezyklen frei von Ermüdungs-schädigung, und ihre intrinsischen Leitfähigkeiten werden sogar während der zyklischen Verformung aufgrund des Kohäsionseffekts verbessert. Bei einer höheren Dehnungsamplitude von 3% können CNT Schichten ihre elektrische Leitfähigkeit bis zu 200.000 Biegezyklen aufrechterhalten, und ihr Versagen ist nur auf die Ermüdung und den Gewaltbruch des Polymersubstrats zurückzuführen, was sich als der limitierende Faktor des ganzen Systems herausstellt. Diese Ergebnisse zeigten, dass die in dieser Studie verwendeten PET-Folien nicht die optimalen flexiblen Substrate für Biegebeanspruchungen sind und alternative Substrate, die bei höheren Dehnungsamplituden (z.B. > 3%) nicht ermüden und brechen, erforderlich sind

Characterization of Nanomaterials: Selected Papers from 6th Dresden Nanoanalysis Symposiumc

This Special Issue “Characterization of Nanomaterials” collects nine selected papers presented at the 6th Dresden Nanoanalysis Symposium, held at Fraunhofer Institute for Ceramic Technologies and Systems in Dresden, Germany, on 31 August 2018. Following the specific motto of this annual symposium “Materials challenges—Micro- and nanoscale characterization”, it covered various topics of nanoscale materials characterization along the whole value and innovation chain, from fundamental research up to industrial applications. The scope of this Special Issue is to provide an overview of the current status, recent developments and research activities in the field of nanoscale materials characterization, with a particular emphasis on future scenarios. Primarily, analytical techniques for the characterization of thin films and nanostructures are discussed, including modeling and simulation. We anticipate that this Special Issue will be accessible to a wide audience, as it explores not only methodical aspects of nanoscale materials characterization, but also materials synthesis, fabrication of devices and applications

Flexible and Stretchable Electronics

Flexible and stretchable electronics are receiving tremendous attention as future electronics due to their flexibility and light weight, especially as applications in wearable electronics. Flexible electronics are usually fabricated on heat sensitive flexible substrates such as plastic, fabric or even paper, while stretchable electronics are usually fabricated from an elastomeric substrate to survive large deformation in their practical application. Therefore, successful fabrication of flexible electronics needs low temperature processable novel materials and a particular processing development because traditional materials and processes are not compatible with flexible/stretchable electronics. Huge technical challenges and opportunities surround these dramatic changes from the perspective of new material design and processing, new fabrication techniques, large deformation mechanics, new application development and so on. Here, we invited talented researchers to join us in this new vital field that holds the potential to reshape our future life, by contributing their words of wisdom from their particular perspective

Ordering of organic molecules on templated surfaces

This thesis describes the controlled growth of molecular nanostructures using modified metallic and semiconductor surfaces. The Ag/Si(lll)-(root3 x root3),the Sn/Cu(100) surface alloy system and the Bi/Si(100) nanolines and (2xn) surfaces were all investigated as suitable substrates for the controlled growth of pentacene, (C22H14) or trimesic acid, (C6H3(COOH)3) organic molecules. The following techniques were used in this study; Scanning Tunnelling Microscopy (STM), Low Energy Electron Diffraction (LEED), Normal Incident X-Ray Standing Waves (NIXSW) and Temperature Programmed Desorption (TPD). The room temperature growth and ordering of trimesic acid on the AgfSi(ll1)-(root3 x root3) surface was investigated. An oblique unit cell was determined and a model proposed for the highly ordered close-packed domains. The discovery of a new submonolayer phase on Sn/Cu(100) and the re-examined known phase are discussed. New models for these reconstructions are proposed. Adsorption of trimesic acid at room temperature on the clean substrate the lowest Sn coverage phase were studied. Two new Sn coverage dependent structures were discovered and bonding schemes in upright and flat orientations are discussed. BifSi(100)-(2xn) surface was exploited as a template for the ordered growth of pentacene, which exhibited orientation specific adsorption. The Bi/Si(100)-(2xn) single domain surface created on vicinal silicon was used to test the suitable of Daresbury 4.2 beamline for NIXSW Imaging experiments and the quality of the results are discussed

Pop-Up Stretchable Sensor Designs Using Multiphysics Modeliing

Stretchable electronic devices are critical for the future of wearable sensor technology, where existing rigid and non-flexible devices severely limit the applicability of them in many areas. Stretchable electronics extend flexible electronics one step further by introducing significant elastic deformation. Stretchable electronics can conform to curvy geometries like human skin which enables new applications such as fully wearable electronics whose properties can be tuned through mechanical deformation. Much of the effort in stretchable electronics has focused on investigation of the optimum fabrication method to make a trade-off between the manufacturing cost and acceptable performance. Here in this thesis a novel pop-up strain sensor design is introduced and tested.This technique is simple to use and can be applied to almost all available materials such as metals, dielectrics, semiconductors and different scales from centi-meter to nanoscale. Using this method three main electronic devices have been designed for different applications. The first category is pop-up antennas that are able to reconfigure their frequency response with respect to the mechanical deformation by out of plane displacement. The second category is pop-up frequency selective surface which similarly can change its frequency behaviour due to applied strain. This ability to accommodate the applied stress by three-dimensional (3D) deformation, making these devices ideal for strain sensing applications such as vapor sensing or on skin mountable sensors. Using the advantage of RFID technology in terms of wireless monitoring, the third category has been introduced which is a pop-up capacitor sensor integrating with an RFID chip to detect finger joint bending that can help those patients who are recovering after stroke. The proposed devices have been modelled using COMSOL Multiphysics and Extensive evaluations of the prototype system were conducted on purpose-built laboratory scale test rigs. Both results are in good correlation which makes them applicable for sensing purposes

Deposition and comparative wear study of thin film coatings

As a part of a group project a magnetron deposition system was designed, constructed and commissioned for producing thin film coatings. Deposition of molybdenum nitride films has been carried out using reactive magnetron sputtering for characterization. The films were characterized in terms of their thickness, hardness and adhesion. The films were reasonably hard but poorly adherent to the steel flats. A wear test rig designed, constructed and commissioned by another research student to carry out impact wear tests was adapted for adhesive wear tests. Progressive wear tests have been done on titanium carbide (5% titanium) coated, titanium nitride coated and uncoated specimens of four substrate materials. The substrate materials are: i) D2 tool steel, ii) D3 tool steel, iii) Vanadis 4 and iv) Vanadis 10. Titanium carbide coatings proved to have good wear resistance but titanium nitride coatings did not. Different coatings imparted different wear resistance to the substrate. The substrate materials have a profound effect on the wear resistance of the coated surfaces