Electronic transport in a carbon nanotube

See also

DFTB-NEGF manual and BAND-NEGF manual

In this tutorial we will be modeling the electronic transport through a carbon nanotube using the NEGF formalism.

/scm-uploads/doc.2022/Tutorials/_images/negf_t1_system_setup.png

Setting up the calculation

The simulation in this tutorial can be performed with either the DFTB engine or the BAND engine. DFTB is computationally faster than BAND, but the results will generally be less accurate.

DFTB
BAND
Start up AMSinput
1. Switch to DFTB: ADFPanel → DFTBPanel
2. Select Task → NEGF
3. Select Model → SCC-DFTB
4. Click on the parameter directory folder and select DFTB.org/3ob-3-1
/scm-uploads/doc.2022/Tutorials/_images/dftb_main_panel.png
Start up AMSinput
1. Switch to BAND: ADFPanel → BANDPanel
2. Select Task → NEGF
/scm-uploads/doc.2022/Tutorials/_images/band_main_panel.png

Note

In this tutorial we use the Non self consistent NEGF method of BAND. This method represents a computationally cheap alternative to the more sophisticated self consistent method, but it can only be used for zero-bias calculations. See the BAND-NEGF Documentation for more details.

Next, we switch to the NEGF panel where we can set up the geometry of our system and specify various NEGF calculation parameters:

Click on MoreBtn next to Task: NEGF to switch to the NEGF panel (or click on Model → NEGF)

We first need an .xyz file defining our lead, that is, a carbon nanotube:

Click here to download the .xyz file carbon_nano_tube_4-4.xyz

Now import the lead:

1. Click on the folder icon next to Lead: this will prompt a file dialog window
2. Open carbon_nano_tube_4-4.xyz (the .xyz file you just downloaded)
3. Set Lead Repetitions to 2
/scm-uploads/doc.2022/Tutorials/_images/negf_t1_select_lead.png

Note

The leads cannot be directly modified in the molecule-editing area. In the next tutorial we will show how to create a leads.

We now need to define our central region. In this tutorial, the central region consist of a nanotube (same as in the lead). We can easily fill in the central region with lead-material with the Fill central region option:

1. Click on Fill
2. Enter 2 in the prompted dialog window and click on OK
/scm-uploads/doc.2022/Tutorials/_images/negf_t1_fill_with_bulk.png

This concludes the geometry-building part of a NEGF calculation. Your system should look like this:

/scm-uploads/doc.2022/Tutorials/_images/negf_t1_geo_ready.png

From the NEGF panel we can change other parameters of the NEGF calculation, but in this tutorial we will use the default values.

Run the calculation and visualize the results

We are ready to run the calculation:

Click on File → Save as…
Run the calculation with File → Run
Wait for the calculation to finish

The results can be visualized with AMSspectra. To start up AMSspectra:

Click on SCM → Spectra…

Here you can visualize:

  • Transmission function \(T(E)\)
  • NEGF-DOS (density of states)
  • Current v.s. Bias Potential (computed from the the zero-bias Transmission function)
In AMSspectra, click on Spectra → Transmission / Transmission+DOS / Current

Note

Energies are relative to the Fermi energy of the lead, i.e in the picture below, 0 eV corresponds to the Fermi energy of the nanotube (first simulation in Simulations work flow)

This is the computed transmission function of our nanotube:

DFTB
BAND
/scm-uploads/doc.2022/Tutorials/_images/dftb_transmission.png
/scm-uploads/doc.2022/Tutorials/_images/band_transmission.png