Exploring Environment-Assisted Quantum Transport of Excitons in Perovskite Nanocrystal Superlattices: Insights from Nature Communications

Nanocrystal superlattices (NCSLs) serve as an exciting platform to explore quantum transport, particularly in disordered materials like colloidal nanocrystal solids. These materials are influenced by static disorder—variations in nanocrystal size and orientation—and dynamic disorder, resulting from factors such as phonon scattering. Understanding these influences is key to optimizing their optical and transport properties.

Take, for example, the perovskite nanocrystals, CsPbBr₃. At low temperatures, they exhibit wave-like, ballistic transport patterns due to minimal dephasing. However, as temperatures rise, thermal fluctuations cause excitons to become localized, leading to random walk diffusion. Interestingly, at intermediate temperatures, there exists a regime called Environment-Assisted Quantum Transport (ENAQT). In this state, weak dephasing can reduce the destructive interference that typically hampers transport, facilitating better exciton movement.

We synthesized CsPbBr₃ nanocrystals using two types of ligands: oleic acid/oleylamine (OA/OAm) and didecyldimethylammonium bromide (DAB). The differences in these ligands affect the coupling strengths and disorder among the nanocrystals. Notably, DAB ligands create closer inter-nanocrystal distances compared to OA/OAm ligands, which influences the electronic interactions between nanocrystals. This variation is crucial because it affects how efficiently the excitons can move. The excitation dynamics also reflect this, with PL (photoluminescence) spectra suggesting that excitons become delocalized when the nanocrystals are assembled into superlattices. This delocalization is a desirable property since it enhances transport capabilities.

When measuring the temperature-dependent PL spectra, we observed that as the temperature increased, the linewidths broadened. This broadening resulted from enhanced scattering caused by phonons, particularly at higher temperatures. Each sample (DAB-1, DAB-2, and OA-1) exhibited unique behavior due to different levels of static and dynamic disorder, which again relate back to the choice of ligands used in their synthesis.

We used two complementary techniques—time-resolved photoluminescence (PL) microscopy and femtosecond transient absorption microscopy—to delve deeper into the exciton transport phenomena across a temperature range from 7K to 298K. The insights from our experiments reveal that the spatial profiles of excitons deviate from typical Gaussian distributions, suggesting they experience non-diffusive, coherent movement instead. This behavior strengthens the case for weakly dissipative regimes, where coherence plays a significant role in exciton mobility.

A particularly striking observation was that the mean squared displacement (MSD) of excitons demonstrated non-monotonic temperature dependence. At lower temperatures, we saw an enhancement of transient, non-diffusive transport, which correlates with lesser dephasing from thermal fluctuations. This behavior suggests a synergistic interaction between coherence and disorder that enhances exciton transport efficiency.

As temperatures rise, exciton behavior shifts, demonstrating the delicate interplay between coherence and disorder. Notably, the transport characteristics vary significantly between our synthesized nanocrystal superlattices and control samples, such as bulk CsPbBr₃ crystals and thin films with randomly distributed nanocrystals. In comparably structured materials, transport relies heavily on dynamic disorder and thermal activation, rather than the coherent transport observed in NCSLs. This indicates that using NCSLs can provide faster transport rates than traditional structures, owing to the unique properties of exciton coupling in these engineered materials.

Ultimately, our findings on ENAQT suggest a turnover temperature where the interplay of dissipation and localization maximizes transport. This non-classical transport behavior means that NCSLs enhance the movement of excitons much more efficiently than typical materials, opening up exciting possibilities for future research and applications in quantum technologies.