Ultrafast symmetry control in photoexcited quantum dots

TL;DR

This paper demonstrates that ultrafast optical excitation can reversibly control the symmetry of colloidal lead chalcogenide quantum dots within picoseconds, revealing a new method for manipulating nanoscale material properties.

Contribution

It introduces a novel ultrafast optical technique to reversibly switch symmetry in quantum dots, combining experimental and theoretical approaches for nanoscale symmetry control.

Findings

Symmetry in lead sulfide quantum dots is restored to centrosymmetric phase within 100 ps after photoexcitation.

Photoexcited electronic carriers suppress lead off-centering, driving symmetry changes.

Symmetry changes are correlated with transient optical properties, enabling optical control of nanoscale symmetry.

Abstract

Symmetry control is essential for realizing unconventional properties, such as ferroelectricity, nonlinear optical responses, and complex topological order, thus it holds promise for the design of emerging quantum and photonic systems. Nevertheless, fast and reversible control of symmetry in materials remains a challenge, especially for nanoscale systems. Here, we unveil reversible symmetry changes in colloidal lead chalcogenide quantum dots on picosecond timescales. Using a combination of ultrafast electron diffraction and total X-ray scattering, in conjunction with atomic-scale structural modeling and first-principles calculations, we reveal that symmetry-broken lead sulfide quantum dots restore to a centrosymmetric phase upon photoexcitation. The symmetry restoration is driven by photoexcited electronic carriers, which suppress lead off-centering for about 100 ps. Furthermore, the change in symmetry is closely correlated with the electronic properties as shown by transient optical measurements. Overall, this study elucidates reversible symmetry changes in colloidal quantum dots, and more broadly defines a new methodology to optically control symmetry in nanoscale systems on ultrafast timescales.