# Simple carbon nanotube device¶

In this tutorial you will build a single-wall carbon nanotube (CNT), relax it along the tube length, and then turn it into a device configuration suitable for nanoscale transport studies.

Attention

This is a two-step procedure:

1. Build and geometry optimize a single CNT unit cell.
2. Elongate the relaxed CNT and turn it into a device configuration.

Note

Geometry relaxing the CNT is particularly important for phonon calculations: Negative phonon frequencies will appear in the phonon band structure if the configuration is not properly relaxed.

## Build and geometry optimize a short CNT¶

Open the VNL Builder , and use Add ‣ From Plugin ‣ Nanotube to create a (6,0) carbon nanotube.

Then send the configuration to the Script Generator by using the botton. In the Scripter, add the following blocks:

• New Calculator.
• Optimization ‣ OptimizeGeometry.

Change the output file to cnt.nc and double-click the New Calculator block to edit the settings:

• Select the ATK-ForceField calculator and the “Tersoff_C_2010” potential.

Note

For ATK-versions older than 2017, the ATK-ForceField calculator can be found under the name ATK-Classical.

Then open the OptimizeGeometry block and edit it:

• Set the maximum force to 0.01 eV/Å.
• Set the maximum stress to 0.001 eV/Å3.
• Remove the checkmark for “z” under “Constrain cell” to allow relaxation of the cell along the periodic direction.

Send the script to the Job Manager , save the script as cnt.py, and run the calculation by clicking . The calculation uses a classical potential, so it should be very fast.

Once finished, you should find the output cnt.nc on the VNL LabFloor. It contains two objects of the type bulk_configuration - the initial (glD000) and the optimized (glD001) CNT structures. You can click an object to highligt it and then use the Viewer to visualize it.

## CNT device configuration¶

Drag and drop the optimized CNT onto the Builder . Then do the following to make it longer:

• In the right-hand side panel, open the Bulk Tools ‣ Repeat plugin.
• Enter a repetition of 10 along C (the tube axis) and click Apply to create a longer CNT.

You are finally ready to convert the configuration to a device and save it:

• Open the Device Tools ‣ Device From Bulk plugin.
• Leave the electrode lengths at default.
• Click OK to create the device.
• Highlight the device configuration, right-click it, and choose Save as in the drop-down menu. You can now save the configuration in either a NetCDF file (cnt_device.nc) or as a Python script (cnt_device.py).
• Note that the CNT device consists of a central scattering region sandwiched between left and right electrodes.

Attention

The instructions above result in a CNT device with a central region that is roughly 43 Å long. This is a somewhat arbitrary choice and it may well be that specific applications require a shorter or a longer central region!

The central region and electrode lengths are easily modified:

• Highlight the device Stash item.
• Use the Device Tools ‣ Central Region Size plugin to change the central region length along the C axis.
• Similarly, the Device Tools ‣ Electrode Size plugin lets you change the electrode lengths.

Tip

What next?
You have created a carbon nanotube device configuration. You can now use it in scientific studies of electron and phonon transport in CNTs.

Thermoelectrics
For example, you can check out the tutorial Thermoelectric effects in a CNT with isotope doping, where phonon and electron transmission spectra, thermal conductance, and the ZT product is calculated for CNTs with and without isotope impurities.

Scripts
For your reference, you can download the CNT device configuration created in this tutorial here: cnt_device.py.