Time-Resolved Photoluminescence Spectroscopy of Semiconductor Nanocrystals and Other Fluorophores

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 notes^4,5,6,7,8. There are also some excellent chapters, reviews, and e-books devoted to a general introduction to TRPL methods^{9,10,11,12,13,14,15}.

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 issue^16,17,18,19. 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

1. Follow all equipment and laser safety procedures for the lab. Always do alignments with the minimum possible laser power. Wear appropriate laser safety glasses.
2. 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: too much light can permanently damage the SPC detector.
3. Make sure to minimize the amount of reflected or scattered laser light that enters the collection optics because this is a major source of artifacts.

2. Setup and pre-alignment

NOTE: Most of these steps should be necessary only if building a new setup.

CAUTION: When doing alignments, wear the appropriate laser safety glasses. Remove reflective personal items such as jewelry or a wristwatch. Damage to equipment can occur if the detector is exposed to too much light, or if you use improper input voltages for your specific equipment.

1. Use a notebook and make a sketch first. Always use at least two mirrors. The mirrors help align the laser beam. For SPC, the setup should look similar to that shown in Figure 1; for slow decays using an acousto-optic modulator (see Discussion), it will look similar to that shown in Figure 2.
2. Have a sample that is either a flat wafer or slide, or a solution in a cuvette. Use a slide or cuvette made of transparent fused silica or quartz. Microscope slides or glass vials should not be used because they have a weak whitish PL background and are UV-absorbing.
3. Try to make the beam lines run along the directions of the optical table. Make sure the tightening knobs for the post holders are easily accessible. Try to keep the beam horizontal as much as possible.
4. If using optical fibers, choose the fibers appropriately. There are different fiber diameters and different wavelength ranges (UV vs. NIR optimized fibers). For weak signals, use a large-diameter fiber. Avoid "mixing and matching" fibers of different types. TRPL may require long fibers due to reflections.
5. If the sample is in a cuvette, use a cuvette holder. If it is a wafer or slide, use a wafer clamp holder.
6. Set up the experiment roughly as shown in Figure 1 or Figure 2. Use the last mirror to make sure the laser beam strikes the sample and lands close to the front of the collection optics.
   NOTE: If the beam strikes near the corner of the cuvette and ends up reflecting all over inside the cuvette, one may get a lot of internal reflections. This can lead to a strange or "messy" TRPL spectrum, especially near the peak.
7. Perform coarse alignment by loosening the knob on the mirror holder and slowly rotating the mirror by hand.
   CAUTION: Do not get any hands in the way of the beam. Be careful of reflections. Weat appropriate laser safety glasses.
8. Perform fine alignment using the alignment screws or knobs on the mirror holder. Make this adjustment by maximizing the PL signal on the computer screen.
9. Block all reflected beams. Make sure all stray beams are accounted for and properly blocked.
10. Never bend the optical fibers to radii smaller than ~20 cm. Otherwise, the fiber can break.
11. Use a longpass filter with a cutoff at least 25 nm longer than the laser wavelength if possible. Interference filters have a forward and backward direction.
12. Obtain a clear, optimized PL spectrum without artefacts. Adjust intensity as appropriate for the SPC detector. If in doubt, use a low PL intensity.

3. TRPL spectroscopy

1. Taking data with the SPC module (fast dynamics)
   1. Ensure that the proper laser-blocking filter and shutter setup is used for the single-photon-counting detector. It will be damaged if it gets too much light. The shutter is closed.
   2. Turn on the laser and adjust the frequency as desired. Turn on the pulse delay generator and set up.
   3. Route the PL to the filter/shutter holder for the SPC detector.
   4. The detector controller and SPC control software should be running as well as cooling if available. Adjust detector gain to a lower value. Power all hardware.
   5. Ensure the sync frequency is correctly registered in the SPCM interface.
   6. Power the PMT (enable outputs). The CFD and TAC should now read a low frequency (dark counts).
   7. Slowly open the shutter to the detector. If you see a saturation warning, close it immediately. Otherwise open it fully.
      CAUTION: Too much light can permanently damage the detector. 
   8. There should now be higher CFD, TAC, and ADC counts. Increase detector gain carefully. Adjust laser power to avoid pile-up.
   9. If the ADC counts are low and no TRPL spectrum is seen, either adjust the delay generator or the sync frequency to bring the TRPL maximum near the left side of the collection window (closer to t = 0).
   10. Perform adjustments to the parameters as described in the main text, until a good PL decay trace in the recorded interval can be observed.
   11. When finished recording, immediately close the shutter and turn off the power to the detector. Turn off the laser. Save data.
2. Taking data with the multichannel scalar (slow dynamics)
   1. Turn on CW laser and acousto-optic modulator control and power.
   2. Open the laser shutter. Set the waveform generator to 1 Hz, appropriate voltage pattern and magnitude (e.g., 0-4 V square wave, 50% duty). Check the output of the AOM. There should be a flashing beam with nearly half the intensity of the main beam. If not, perform a full alignment of the AOM.
   3. Using the iris, ensure that only the bright, flashing beam is aligned onto the sample. Increase the frequency to the desired value (e.g., 200 Hz). Check the PL = using a spectrometer, as described previously.
   4. Power the detector. Run the MSC software. Set the MSC software according to procedure. Choose your timesteps.
   5. Route the PL to the detector input. Make sure the appropriate filter is used.
   6. Power the PMT, and then slowly open the shutter as in step 3.1.7. Close the shutter immediately if saturation warning appears. If so, weaken the PL signal.
   7. Collect data. Immediately close the shutter and disable the power to the PMT when finished. Close the laser shutter. Save the data.

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...

Materials

Name	Company	Catalog Number	Comments
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