|
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.
The QuantumWise software platform, with the first-principles engine Atomistix ToolKit (ATK) and the graphical user interface Virtual NanoLab (VNL) 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.
The unique feature of ATK to calculate ballistic tunneling current in nanostructures enables users to
- calculate the conductance of graphene nanoribbons of various edge types and shapes
- compute the current/voltage characteristics when a finite bias is applied to the system
- study spin transport properties
- simulate the influence of a gate, to compute transistor characteristics
- investigate the details of the transport mechanisms
In addition, ATK is a very convenient tool for studies of basic properties of graphite/graphene, such as
- band structure of doped graphene structures
- dependence of the electronic structure (and transport properties) on the number of layers (bilayer, trilayer graphene, etc)
- and many other things!
The properties of graphene are closely related to carbon nanotubes, which is another very active application area for ATK.
Since ATK uses first-principles methods for the computations, one is not limited to just carbon-based materials. Very exciting properties, similar and/or different compared to graphene, have been reported in hexagonal monolayers of
- boron-nitride (B-N)
- zinc-oxide (Zn-O)
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 efficient and easy.
Examples and references
Below we illustrate some of the graphene structures that users of Atomistix ToolKit have studied with the software. To view all references to papers on graphene studied with ATK, see the publication list.
|
Bilayer graphene nanoribbons with armchair edges
|
K.-T. Lam & G. Liang, Applied Physics Letters 92, 223106 (2008)
Dependency of energy bandgap of bilayer armchair graphene nanoribbons on their widths, interlayer distance, and edge doping concentration of boron/nitrogen was investigated. The bandgap is highly sensitive to the interlayer distance, indicating a possible application in tuning the gap. Edge doping reduces the gap by a smaller amount than for monolayer ribbons.
|
|
Graphene flakes
|
H. Sahin & R.T. Senger, Physical Review B 78, 205423 (2008)
Study of structural, electronic, and transport properties of hydrogen-terminated short graphene nanoribbons (graphene flakes) and their functionalization with vanadium atoms. A spin-polarized current can be produced by exploiting the spatially separated edge states using asymmetric nonmagnetic contacts. Functionalization of the graphene flake with magnetic adatoms such as vanadium also leads to spin-polarized currents even with symmetric contacts. |
|
Resonant-tunneling double-barrier system
|
H. Sevincli, M. Topsakal & S. Ciraci, Physical Review B 78, 245402 (2008)
Junctions of armchair graphene (or commensurate graphene/boron-nitride) nanoribbons of different widths form quantum well structures. Calculation of the transmission coefficient through a double barrier resonant tunneling device, formed from a finite segment of such a multiple quantum well structure placed between metallic electrodes, yields resonant peaks which can be identified with electronic states confined in the well.
|
|
T-shaped graphene junctions
|
F. OuYang et al., Nanotechnology 20, 055202 (2009)
Intrinsic transport properties and effective boron or nitrogen doping of T-shaped and crossed junctions based on "shoulders" (zigzag graphene nanoribbons) joined with "stems" (armchair ribbons) are studied. The I-V characteristics of the pure-carbon T-shaped junctions were shown to have metallic behavior, and the current of the junction strongly depends on the height of the stem. The conductance of the devices is found to depend sensitively on their geometric structures and be controlled by selective doping.
|
|
Zigzag graphene nanoribbon
|
Z. Li et al., Physical Review Letters 100, 206802 (2008)
Although all zigzag edge graphene nanoribbons have similar metallic band structure, they show distinctly different transport behaviors under bias voltages, depending on whether they are mirror symmetric with respect to the midplane between the two edges. Asymmetric ZGNRs behave as conventional conductors with linear current-voltage dependence, while symmetric ZGNRs exhibit unexpected very small currents with the presence of a conductance gap around the Fermi level. This difference is revealed to arise from different coupling between the conducting subbands around the Fermi level, which is dependent on the symmetry of the systems.
|
|
Z-shaped armchair/zigzag/armchair graphene nanoribbon junction
|
H. Ren et al., Chinese Journal of Chemical Physics 20, 489 (2007)
Investigation of electronic and transport properties of a Z-shaped graphene nanoribbons heterojunction. The robust quantum confinement effect in the junction can be used to design a quantum dot, which can be realized regardless of doping impurity, edge chemical modification, and the length of the junction. The spatial distribution and the number of confined states are tunable.
|
|
Z-shaped zigzag/armchair/zigzag graphene nanoribbon junction
|
Q. Yan et al., Nano Letters 7, 1469 (2007)
Electronic devices built on patterned graphene nanoribbons (GNRs) can be made with atomic-perfect-interface junctions and controlled doping via manipulation of edge terminations. GNR field effect transistors can achieve high performance levels similar to those made from single-walled carbon nanotubes, with ON/OFF ratios on the order of 103-104, subthreshold swing of 60 meV per decade, and transconductance of 9.5 × 103 Sm^-1.
|
|
Selectively edge-doped nanoribbon
|
B. Huang et al., Applied Physics Letters 91, 253122 (2007)
A metal-semiconductor transition is induced by substitutional doping of nitrogen or boron at the edges of a graphene nanoribbon with zigzag edges. A field effect transistor consisting of a metal-semiconductor-metal junction can then be constructed by selective doping of the ribbon edges.
|
Some further details
There are several specific features of ATK that makes it especially well suited for simulations of graphene:
- Full DFT (LDA and PBE/revPBE) simulation of electronic structure and transport properties, also spin-polarized
- Localized basis sets with compact support (SIESTA type) makes it effortless to describe the vacuum surrounding the ribbon, and also provides an accurate description of doping atoms without computational overhead
- The localized basis set also means ATK can handle comparatively large systems (more atoms), in fact up to 1,000 atoms can relatively easily be simulated on a laptop
- Parallelized code with linear speedup for transmission and current calculations
|
