Graphene studies with ATK

One of the hottest and latest research trends in nanotechnology is studies of graphene for a multitude of applications within nanoelectronics and other related areas. Researchers around the world, both in academic and industrial R&D departments, are using QuantumWise software extensively to investigate this material and develop future devices based on various types of graphene structures.

Atomistix ToolKit (ATK) allows researchers to focus on the relevant points for their projects, namely the investigation of the electrical properties of novel device structures, rather than spending time writing their own quantum-mechanical codes for the simulation, or struggling with data import/export to visualize the results.

Over 200 scientific articles have been published with ATK on graphene-related topics, the first one dating back as early as 2007.

Features

The effects observed in graphene structures are often an effect of the shape, rather than the detailed chemistry. Hence even simple methods can predict many properties accurately. On the other hand, certain applications like gas sensing, or metal/graphene contacts, require a detailed quantum-chemical description of the interactions between molecules or other materials, and graphene. Having access to a wide variety of methods in a single tool makes working with ATK efficient and flexible, since you don't have to spend time learning several interfaces, transferring structures between different input formats, etc.

All methods in ATK are accessed via a common interface, making it very convenient to switch between different methods to compare the results (accuracy etc). Depending on the selected method, calculations can be performed on structures with several hundred (DFT/DFTB and Hückel) to tens of thousands (tight-binding) of atoms. These methods can be used to compute a wide variety of properties of both simple periodic graphene and more complex device-like geometries.

Electronic structure properties like band structure, density of states, real-space density, Bloch states, and other relevant quantities Transport properties at finite bias via non-equllibrium Green's functions Current-voltage (I-V) curve Transmission spectrum Conductance Voltage drop Transmission eigenstates and pathways Transistor characteristics Insert metallic gates and dielectric screening regions, and compute the on/off ratio subthreshold swing Thermoelectric properties Seebeck coefficient Thermionic emission current Optical properties Absorption spectrum Dielectric constant Refractive index Mechanical properties

All of the above properties can also be computed with added collinear spin-polarization, thus you can compute e.g. spin currents and magnetoresistance.

See the complete list of features in ATK for more details!

Graphical user interface

Setting up the geometric structures of the systems to be studied is easy in the graphical user interface (GUI) Virtual NanoLab (VNL), which has dedicated tools to generate and manipulate graphene sheets, nanoribbons, etc. It is for instance simple to passivate the edges of a ribbon, or interactively introduce defects, dopants or vacancies. The GUI also provides a user-friendly interface to set up the numerical parameters of the calculation, and to plot and analyze the results.

Applications

Using the features outlined above, researchers have applied ATK to many different structures involving or related to graphene:

• graphene nanoribbons (GNR)
• infinite graphene sheets
• crossed nanoribbons, junctions of various shapes (T-shaped, Z-shaped)
• imperfections in the form of constrictions, V-shaped notches, vacancies, Stone-Wales defects, divacancies, etc
• monolayer, bilayer, trilayer graphene
• triangular flakes and other finite structures
• corrugated sheets and ribbons
• strained or disordered structures

One is not limited to pure-carbon systems; many important device ideas come from introducing dopants in GNRs, and functionalization by covalently or non-covalently bonded molecules and ad-atoms can lead to important modifications of the electronic properties that can be used to construct tailor-made device characteristics. In many device structures the source and drain electrodes are metal surfaces (gold, nickel, aluminium, etc), and junctions can also be formed with boron-nitride ribbons or sheets, carbon nanotubes (CNT), nanowires, atomic chains, and other nanoscale structures.

Carbon is not the only element that can form regular, hexagonal monolayer structures. Examples of interesting such materials that can be studied with ATK, and which have both very different and similar properties, are boron-nitride (BN), zinc-oxide (ZnO), silicene (Si), and molybdenite (MoS2). We can in this context also mention the closely related materials graphane and graphone. Moreover, the properties of graphene are closely related to carbon nanotubes, which is another very active application area for ATK.

Examples of graphene device ideas studied with ATK

• Bipolar field-effect transistors, diodes, switches, and rectifiers
  • Edge doping/vacancies and other types of defects can be used to obtain a p-n heterojunction
  • Shape control can also be used to create a metal-semiconductor-metal junction, resonant tunneling diode structures, etc
  • Negative-differential resistance (NDR) can be observed in many different geometries
  • Metallic gates can be introduced to control the effective potential in the switching region
• Spintronics devices
  • Spin filter effects in graphene nanoribbons, induced by impurities (magnetic or non-magnetic), or edge defects
  • Spin logic gates
  • Magnetic tunnel junctions (MTJ), for instance based on metal-graphene junctions with nickel
• Sensors
  
  • Detecting trace amounts of gases - defect sites with dangling bonds form sites with strong binding, even for small molecules like CO, NO or NO₂
  • DNA sequencing or base pair detection
• Thermoelectric (caloritronic) devices
  • Thermally induced currents
  • Themionic emission, band-to-band tunneling
  Nanoelectromechanical systems like ultra-sensitive force sensors based on bilayer graphene

Each of these topics has been studied in one or more published articles. Abstracts and links to the full text can be found in the searchable ATK publication list.

Tutorials

If you are interested in studying graphene with ATK, an excellent starting point will be our tutorials. These cover a range of topics, from basic band structure calculations of graphite to transport analysis of graphene nanoribbons. Using the graphical user interface, sometimes combined with NanoLanguage scripts, makes it efficient and easy to set up even advanced geometries like a z-shaped junctions between armchair/zigzag nanoribbons efficiently and easy.