Time-Resolved and Temperature-Dependent Fractional Amplitude Contributions to the Broadband Emission of CdSe Quantum Dots

Quinton Rice,Jaetae Seo,Sangram Raut,Kyle Burney,Ignacy Gryczynski,William Yu,Bagher Tabibi,Zygmunt Gryczynski

doi:10.3390/cryst11111284

Abstract

The broadband spontaneous emission of excitons in CdSe quantum dots (QDs) is of great interest for the spectral imaging of living organisms or specific substances in the visible spectral region as well as in the biological optical window near the infrared spectral region. Semiconductor QDs that are near the bulk Bohr radius exhibit wide spectral tunability and high color purity due to quantum confinement of excitons within the dot boundary. However, with reducing dot size, the role of the surface-trapped state increases. The temperature-dependent photoluminescence (PL) confirms this with a ~3:1 emission intensity decrease from the surface-trapped state compared to the band edge. Large crystal irregularity, dangling ions, and foreign molecules can introduce new electronic transitions from surface-trapped states that provide broad spontaneous emission in the spectral region from visible to near IR in addition to the band edge emission. The time-resolved PL analyzed the fractional contributions of band edge, surface-trapped states, and possible intermediate trapped states to the broad spectral emission in order to characterize the CdSe QDs.

Highlights

Cadmium chalcogenide (Te, Se, and S) semiconductor quantum dots (QDs) typically possess high color purity, wide optical tunability, and large quantum yield in the visible and near infrared spectral region, which allows for various applications in photonics, LED and solar cell development, as well as optical sensing and bio-imaging [1,2,3,4,5,6,7,8,9,10,11,12]
It is widely known that the bandgap is the main emission site for CdSe QDs; other radiative transition may occur on the surface of the nanocrystal due to atomic vacancies, local lattice mismatches, adsorbates at the surface, dangling bonds, or imperfect crystallization during the time-sensitive synthesis process [14,15,16]
To enhance the quality and remove non-radiative decay in CdSe QDs, typically the surface defects are passivated with neutral ligands such as trioctylphosphine oxide (TOPO) or capped with high bandgap materials such as ZnS, which reduce non-radiative recombination and result in narrow spectral emission [17]

Summary

Introduction

Cadmium chalcogenide (Te, Se, and S) semiconductor quantum dots (QDs) typically possess high color purity, wide optical tunability, and large quantum yield in the visible and near infrared spectral region, which allows for various applications in photonics, LED and solar cell development, as well as optical sensing and bio-imaging [1,2,3,4,5,6,7,8,9,10,11,12]. To enhance the quality and remove non-radiative decay in CdSe QDs, typically the surface defects are passivated with neutral ligands such as trioctylphosphine oxide (TOPO) or capped with high bandgap materials such as ZnS, which reduce non-radiative recombination and result in narrow spectral emission [17]. Without such passivators or capping agents, the dangling bonds are free to exist and, surface defects are highly influential to the overall photoluminescence (PL) at longer wavelengths. While the temperature dependent bandgap is well known for bulk and QDs of CdSe, the dynamics of the surface-trapped state is less familiar

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