Quantum Dot Performance Limited by Atomic Structure, Not Just Defects

Research identifies performance-limiting defects in indium phosphide and gallium phosphide quantum dots arising from fully-coordinated atoms with distorted geometries, termed structural traps. These anion-centered hole traps persist despite surface passivation and originate from bond stretching or angular distortion, potentially influencing quantum dot dynamics.

Quantum dots – semiconductor nanocrystals exhibiting quantum mechanical properties – hold considerable promise for applications ranging from displays to biomedical imaging. However, the performance of quantum dots fabricated from group III-V materials – such as indium phosphide and gallium phosphide – is often limited by defects that trap charge carriers, reducing efficiency. Recent research identifies a previously underappreciated source of these defects: ‘structural traps’ arising not from missing atoms on the surface, but from fully coordinated atoms exhibiting distorted geometries. Ezra Alexander, Alexandra Alexiu, Matthias Kick, and Troy Van Voorhis, all from the Massachusetts Institute of Technology, detail their findings in a study titled ‘Structural Hole Traps in III-V Quantum Dots’, presenting evidence for these anion-centered traps and proposing a molecular orbital explanation for their formation via bond stretching and angular distortion. Their work suggests that conventional surface passivation techniques may be insufficient to eliminate these defects, necessitating alternative strategies to rigidify the quantum dot structure and unlock the full potential of III-V materials.

Structural Distortions Limit Performance of III-V Quantum Dots

Recent research identifies a previously unrecognised source of performance-limiting defects within indium phosphide (InP) and gallium phosphide (GaP) quantum dots (QDs). The findings challenge conventional understanding by demonstrating that defects arise not solely from surface imperfections, but from distortions within the fully coordinated atomic structure – termed ‘structural traps’.

Quantum dots are semiconductor nanocrystals exhibiting quantum mechanical properties dependent on their size. They are promising materials for applications including displays, solar cells and biomedical imaging. However, their performance is often limited by defects which trap charge carriers, reducing efficiency.

The study identifies these structural traps as arising from bond distortions – specifically, bond stretching and angular deviations resulting in a ‘see-saw’ like geometry within the QD lattice. These distortions create localised states that act as traps for holes – the absence of an electron, behaving as a positive charge carrier – hindering their movement and reducing device performance.

Critically, these traps demonstrate insensitivity to ligand passivation. Ligand passivation involves coating the QD surface with organic molecules to reduce surface defects. The research demonstrates that simply addressing surface states does not eliminate these internal structural distortions. This finding departs significantly from established defect mitigation strategies.

The methodology employed combines ab initio density functional theory (DFT) calculations – a quantum mechanical modelling approach – with machine learning (ML) techniques. DFT calculations were used to model the atomic structure and electronic properties of the QDs, while ML algorithms – including Support Vector Machines (SVMs), Random Forests, Synthetic Minority Oversampling Technique (SMOTE), Adaptive Synthetic Sampling Approach (ADASYN), and Uniform Manifold Approximation and Projection (UMAP) – were applied to analyse the complex data generated and identify key structural features correlated with trap formation.

The findings suggest that enhancing structural rigidity is paramount for improving QD performance. Traditional core-shell passivation strategies, which focus on surface modification, are insufficient unless they also address and stabilise the underlying atomic structure. The transient nature of these distorted geometries also suggests the possibility of dynamic behaviour within the QDs, potentially opening avenues for investigation into novel optoelectronic properties.

This research offers insight into the observed performance disparity between III-V QDs (such as InP and GaP) and II-VI/IV-VI QDs (such as cadmium selenide or lead sulphide). The findings suggest that structural stability is a key differentiating factor, with III-V materials exhibiting a greater propensity for these internal distortions.

In essence, this work shifts the focus from surface passivation to internal structural optimisation as a critical pathway for enhancing the performance of InP and GaP quantum dots, providing a new framework for understanding and addressing defects in these materials.