Quantum dots exhibit extraordinary electronic properties and are often referred to as artificial atoms. They allow charge control down to atomic scales and are used in the world's smallest transistor – a quantum dot transistor built of seven atoms.

As modern electronics reaches the nanoscale, quantum effects and the "atomic granularity" of materials reveal themselves in device behaviour. Simulating quantum and atomistic effects for realistic devices is computationally challenging, and thus is often narrowed to unrealistically small structures.

Multimillion-atom system

With supercomputing and advanced simulation tools Purdue engineers try to model real world nanodevices – in this study, the optical emission spectra of an InGaAs quantum dot molecule fabricated by German scientists at the Walter Schottky Institute.

The simulation was run using NEMO 3D, one of the group's nanoelectronic modeling tools, which can evaluate structures with more than 50 million atoms through parallelized computation. (Open-Source NEMO 3D based simulation tools are available at https://nanohub.org/groups/nemo_3d_distribution).

"The excellent agreement of the simulation with experimental data gives us faith in our models to explore beyond the experimentally observed range," commented Muhammad Usman and Yui-Hong Matthias Tan, affiliates of the Nanoelectronic Modeling Group at Purdue University. "Trying to explain new physics and using modeling to guide experimental studies are some of the main objectives in our group."

Spectroscopic approach

The team not only reproduced experimental data, but also demonstrated a spectroscopic method to extract information about the quantum dots' geometry and energy level separations from simulated emission spectra. A varying electric field is modeled to probe through the quantum dots' energy levels and excitonic anti-crossings in the emission spectra.

The study is also the first to model atomistically both linear and quadratic piezoelectric effects in a quantum dot molecule. Contrary to previous studies, the team demonstrates that piezoelectric effects are crucial in reproducing and understanding experimental emission spectra in strained quantum dot molecules.

Additional details can be found in the journal Nanotechnology.

•  Images created by Insoo Woo in Prof. David Ebert's Visualization Laboratory at Purdue University.