Time-Resolved Photoluminescence Spectroscopy of Semiconductor Nanocrystals and Other Fluorophores

Published: August 04, 2022

doi:

¹Department of Physics,University of Alberta

Summary

This paper presents an experimental how-to on time-resolved photoluminescence. The hardware used in many single photon-counting setups will be described and a basic how-to will be presented. This is intended to help students and experimenters understand the key system parameters and how to correctly set them in time-resolved photoluminescence setups.

Abstract

Time-resolved photoluminescence (TRPL) is a key technique for understanding the photophysics of semiconductor nanocrystals and light-emitting materials in general. This work is a primer for setting up and conducting TRPL on nanocrystals and related materials using single-photon-counting (SPC) systems. Basic sources of error in the measurement can be avoided by consideration of the experimental setup and calibration. The detector properties, count rate, the spectral response, reflections in optical setups, and the specific instrumentation settings for single photon counting will be discussed. Attention to these details helps ensure reproducibility and is necessary for obtaining the best possible data from an SPC system. The main aim of the protocol is to help a student of TRPL understand the experimental setup and the key hardware parameters one must generally comprehend in order to gain useful TRPL data in many common single-photon-counting setups. The secondary purpose is to serve as a condensed primer for the student of experimental time-resolved luminescence spectroscopy.

Introduction

Time-resolved photoluminescence (TRPL) is an important and standard method for studying the photophysics of luminescent materials. TRPL measurement systems can be open setups constructed by the experimenter or they can be self-contained units purchased directly from a manufacturer. Open setups are considered superior to "closed-box" TRPL units because they permit more experimental control and additional ways to collect useful data; however, they demand a more complete understanding of the measurement. TRPL is widely employed in the development of luminescent devices and should always be reported along with the basic emission spectrum of semiconductor nanocrystals and other light emitting materials. There are many methods for doing TRPL; this primer focuses on single photon counting systems.

Before starting, it is important to acknowledge a number of previous works. First, the Principles of Fluorescence Spectroscopy by Joseph Lakowicz¹ is a large compendium containing a chapter on TRPL methods. Ashutosh Sharma's Introduction to Fluorescence Spectroscopy contains a now somewhat dated chapter on time- and phase-resolved fluorimeters² used principally by chemists and biologists. Fluorescence Spectroscopy: New Methods and Applications³ remains valuable although it is over 20 years old. The most recent information and advances can be found in handbooks and technical notes⁴^,⁵^,⁶^,⁷^,⁸. There are also some excellent chapters, reviews, and e-books devoted to a general introduction to TRPL methods⁹^,¹⁰^,¹¹^,¹²^,¹³^,¹⁴^,¹⁵.

Single photon counting (SPC) methods are common and widely employed, but there are several concepts that students of fluorescence spectroscopy should learn in order to take good data. The principles herein are general and applicable to a wide range of SPC experimentation. Of course, once the data has been collected, the fitting algorithms and methods are another essential art. The TRPL model fitting is critically important and is often done improperly despite the fact that many previous works have specifically focused on this particular issue¹⁶^,¹⁷^,¹⁸^,¹⁹. The present work, however, focuses primarily on experimental aspects of TRPL.

The rationale for this work is to develop a comprehensive guide toward performing TRPL with common single-photon-counting (SPC) modules. Because these systems are technically complicated, a good understanding of the basic experimental variables is important for optimizing the data collection and minimizing the appearance of avoidable artefacts. While techniques such as optical Kerr gating and equipment such as streak cameras present special opportunities for ultrafast TRPL¹⁵, recent technical developments in the field of SPC have made nanosecond and sub-nanosecond TRPL readily accessible to almost any experimental optics lab. SPC additionally offers speed and resolution improvements over older methods such as photodiode-oscilloscope combinations.

Protocol

1. Preparation Follow all equipment and laser safety procedures for the lab. Always do alignments with the minimum possible laser power. Wear appropriate laser safety glasses. Check the PL spectrum from the sample before connecting the output to the SPC setup. Make sure that the spectrum looks as expected and that none of the excitation laser light is present. The PL may have to be tuned down by weakening the excitation source or using neutral density filters. NOTE: Warning: …

Representative Results

A standard SPC decay curve is shown in Figure 3. The initial rise was shifted so that the peak corresponds to zero time (this is not the case in the raw data due to the electronic and optical delays). The signal-to-background ratio is about 100 because this sample has a long-lived but weak phosphorescence. A weak reflection is clearly observable on the log scale, which occurs about 50 ns after the main TRPL peak. This is due to reflections inside the 5-meter-long optical patch fiber, as repo…

Discussion

There are several important user-controlled parameters in any SPC setup that must be understood by the user. These parameters will explain the limitations of the SPC method for TRPL, allow the user to troubleshoot the setup more easily if something goes wrong, and help to understand the critical steps that are effectively required for good data collection. Moreover, different samples will often require different system settings – in other words, one cannot have a single procedure for all possible SPC decay traces. The se…

Disclosures

The authors have nothing to disclose.

Acknowledgements

The Natural Sciences and Engineering Research Council of Canada provides funding for this research. Thanks to Xiaoyuan Liu for performing the fit in Figure 3 and Dundappa Mumbaraddi for providing the rare-earth-doped perovskite sample. Thanks to Julius Heitz for making Reference²⁰ available.

Materials

AOM	Isomet	1260C
Laser	Alphalas	Picopower
Laser	Coherent	Enterprise
MCS	Becker-Hickl	PMS-400
PMT	Becker-Hickl	HPM100-50
PMT	Hamamatsu	H-7422
SPCM	Becker-Hickl	EMN130

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