Monolayer-Defined Flat Colloidal PbSe Quantum Dots in Extreme Confinement

TL;DR

This paper investigates monolayer-defined flat PbSe quantum dots, revealing their electronic structure, optical properties, and synthesis control, advancing their potential for telecom-band photonic applications.

Contribution

It provides detailed experimental and theoretical insights into the structure, electronic states, and optical behavior of monolayer PbSe quantum dots, enabling precise property tuning.

Findings

Quantum dots exhibit high quantum yield photoluminescence in the telecom band.

Structural and electronic properties depend on thickness and size.

Surface passivation can slow down ripening and fusion processes.

Abstract

Colloidal two-dimensional lead chalcogenide nanocrystals represent an intriguing new class of materials that push the boundaries of quantum confinement by combining a crystal thickness down to the monolayer with confinement in the lateral dimension. In particular flat PbSe quantum dots exhibit efficient telecommunication band-friendly photoluminescence (1.43 - 0.83 eV with up to 61% quantum yield) that is highly interesting for fiber-optics information processing. By using cryogenic scanning tunneling microscopy and spectroscopy, we probe distinct single layer-defined PbSe quantum dot populations down to a monolayer with in-gap state free quantum dot-like density of states, in agreement with theoretical tight binding calculations. Cryogenic ensemble photoluminescence spectra reveal mono-, bi-, and trilayer contribution, confirming the structural, electronic and theoretical results. From larger timescale shifts and ratio changes in the optical spectra we infer Ostwald ripening in solution and fusing in deposited samples of thinner flat PbSe quantum dots, which can be slowed down by surface passivation with PbI2. By uncovering the interplay between thickness, lateral size and density of states, as well as the synthetic conditions and post-synthetic handling, our findings enable the target-oriented synthesis of two-dimensional PbSe quantum dots with precisely tailored optical properties at telecom wavelengths.