Dislocations in Graphene Alison Richardson Michael Kennedy Christopher Dismuke Abdulrahman Alabdulmohsin

Overview Introduction Why Graphene? Why study dislocations in Graphene? Basic Principles Work performed Experiments Conducted Stone Wales Defect Results

Introduction The movement of dislocation in crystals can determine: the strength of a material under a load the way it accommodate strain 3D analysis, High-resolution transmission electron microscopy (TEM), tracks the movement of dislocations Transmission Electron Microscopy is used to study dislocation in graphene TEM produces images like the above depicting molecular structure Image Source: http://hitachi-hta.com/products/electron-microscopes-and-focused-ion-beam/transmission-electron-microscopes/h-9500-300kv-te

Why Graphene Was Chosen Graphene is a two dimensional material Studying Graphene could simplify studying the movement of dislocations Using sophisticated TEM technique we can: see dislocations in graphene create dislocations in graphene move dislocations in graphene The results can improve our theoretical understanding on the effects of defects on mechanical properties Graphite Graphene Nanotube Fullerene Image Source: http://graphene.nus.edu.sg/content/graphene

Why Study Dislocations in Graphene? Understanding how dislocations deform helps describe Elasticity Plasticity in graphene Defects and strain can lead to spin and magnetism Extends graphene’s electronic applications into spin-based technology Example For the logic operations electrons move through the graphene use its spin state to compare the information held in the individual magnetic electrodes Spin caused by dislocation Image Source: http://www.kurzweilai.net/spin-based-magnetologic-gate-to-replace-silicon-chips

Example: Graphene Biosensors Graphenes large surface area and strong bonding enables electrical detection of pathogenic species Source: http://web.ics.purdue.edu/~chen658/research.htm

Basic Principles Color variation shows stresses in a dislocation pile-up Image Source: http://www.geol.ucsb.edu/faculty/hacker/geo102C/lectures/part11.html In graphene, electronic properties can have noticeable effect on the mechanical properties Strains may cause strong pseudo-magnetic fields Landau level vacancies Point defects produce paramagnetism Low-voltage transmission electron microscopy (TEM) can determine the lattice structure of graphene with high contrast and minimal damage

Pseudo-Magnetic Fields Pseudovector: a quantity that transforms like a vector under a proper rotation, but gains an additional sign flip under an improper rotation such as a reflection Examples of pseudovectors include the magnetic field, torque, vorticity, and the angular momentum magnetic field is not reflected, but reflected and reversed The position of the wire and its current are vectors, but the magnetic field is a pseudovector.[1] position and current of the wire are reflected Image Source: http://en.wikipedia.org/wiki/File:BIsAPseudovector.svg

Landau Level Vacancies Landau quantization in quantum mechanics is the quantization of the cyclotron orbits of charged particles in magnetic fields The charged particles can only occupy orbits with discrete energy values, called Landau levels The Landau levels are degenerate, with the number of electrons per level directly proportional to the strength of the applied magnetic field Landau quantization is directly responsible for oscillations in electronic properties of materials as a function of the applied magnetic field Using scanning tunnelling spectroscopy - a spatially resolved probe that interacts directly with the electrons - scientists at institutions including the University of Warwick and Tohoku University have revealed the internal ring-like structure of these Landau Levels at the surface of a semiconductor Image Source: http://www2.warwick.ac.uk/newsandevents/pressreleases/first_images_of/simull1.png

Point Defect Production of Paramagnetism Paramagnetism is the tendency of magnetic dipoles to align with an external magnetic field. Orientation in paramagnetic material Electric Field Applied Electric Field Removed Image source: https://www.boundless.com/physics/magnetism/applications-magnetism-2/paramagnetism-and-diamagnetism-2/

Experiment Prepare sample with chemical vapor deposition onto copper foils Transfer to silicon nitride grids for TEM with 2 µm holes High Resolution TEM Spherical Aberration Correction Monochromation Generate strain field map Strain field map resulting from experimental procedure Monochromation applied to (A) to produce (C) - (F) Source: http://www.sciencemag.org/content/337/6091/209/F2.large.jpg

Work performed Electrons used for imaging are accelerated at low voltages rotate C-C bond & create defects Image source: http://www.kintechlab.com/solutions/nanotechnology/nucleation-and-growth-of-carbon-nanostructures/ Image source: http://en.wikipedia.org/wiki/User:Chem511grp1f09/Sandbox The maximum energy transferred from the beam has to be less than 17 eV/90 kV, so the electron beam will not knock atoms off the graphene lattice, but will knock atoms off defective sites

Formation of Stone-Wales defect (SWD) 9.2 eV causes a 90(o) C-C bond rotation transforming four hexagons to two pentagons and two heptagons SWD is two dislocations forming a dipole the dipole is resulted from equal length and antiparallel Burger vectors reverting back to the pristine, stress-free lattice, 4 eV has to be activated. This energy is less than the maximum energy the TEM can transfer. Stone-Wales transformation resulted from the application of 9 eV energy Stone-Wales transformation

Applying Shear Stress Applying enough shear stress results on splitting a SWD into its two component dislocations. These two components then drift apart. The dislocations have zero overall Burger vector SWD drifting apart with zero overall burger vector Motion along the Burger vector SWD Image source: http://research.che.tamu.edu/groups/Seminario/Materials_CHEN313_Spring_2013/special%20assigment/Materials_G02_Driving%20Dislocations%20in%20Graphene.pdf

Motion of Dislocation It was observed that one dislocation moved along the direction of its burger vector The motion require only atomic rotation (about 5eV) SWD drifting apart with zero overall burger vector Motion along the Burger vector SWD

Climbing of Dislocation Then the same dislocation climbed perpendicular to its burger vector The climbing involve removal of two atoms (about 9-12 eV) climbing perpendicular to the burger vector SWD drifting apart with zero overall burger vector Motion along the Burger vector SWD

Results Edge dislocations results in structural deformation Close proximity of paris of dislocations magnifies deformation Electron beam introduced localized variations Double Wien filter produced high resolution capable of mapping location of single carbon atom www.sciencemag.org

Conclusion The results prove that the experiment and the theory are the same It also offers understanding of plastic deformation in nanoscale materials Graphene could be used as a reference for quantitative theory of irradiation-driven dislocation dynamics

Assessment of Work One of the most accurate work done on graphene Provides opportunities for diverse applications in studies of material deformations Creates opportunities for wide array of applications to new technologies. Uses of Graphene identified by this work: Cell phone screens http://cdn.physorg.com/newman/gfx/news/hires/grapheneOLED.jpg Graphene-based transistors http://www.nature.com/nature/journal/v467/n7313/carousel/nature09405-f1.2.jpg Graphene solar cells http://images.gizmag.com/inline/graphene-electrode-1.jpg

Further Research Controlling electronic properties in graphene through strain engineering Experiments on how to make appropriate dislocations and control their motion Study dislocation in other materials Use HRTEM to study dislocation in other materials Image source: http://upload.wikimedia.org/wikipedia/vi/6/6f/HRTEM.jpg Image Source: http://hitachi-hta.com/products/electron-microscopes-and-focused-ion-beam/transmission-electron-microscopes/h-9500-300kv-te

References 1) Warner, Jamie H. "Dislocation-Driven Deformations in Graphene." Science 10th ser. 337.1126 (2012): 209-12. Print. 2) Bonilla, Luis L., and Ana Carpio. "Driving Dislocations in Graphene." Science 10th ser. 337.1126 (2012): 161-62. Print.