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Materials Science

Time-Resolved Photoluminescence (TRPL)

From picoseconds to milliseconds

The fluorescence (or more generally the photoluminescence) lifetime is an intrinsic characteristic of a luminescent species that can provide insight into the species excited state dynamics. Time-Resolved Photoluminescence (TRPL) is the tool of choice for studying fast electronic deactivation processes that result in the emission of photons, a process called fluorescence. The lifetime of a molecule in its lowest excited singlet state usually ranges from a few picoseconds up to nanoseconds. This fluorescence lifetime can be influenced by the molecular environment (e.g., solvent, presence of quenchers (O2), or temperature) as well as interactions with other molecules. Processes like Förster Resonance Energy Transfer (FRET), quenching, solvation dynamics, or molecular rotation also have an effect on the decay kinetics. Lifetime changes can therefore provide information about the local chemical environment or insights into reaction mechanisms.

Some species such as metal-organic complexes, inorganic crystal structures, semiconductors and new types of hybrid materials have emission lifetimes ranging from nano- to micro- or even up to milliseconds. In this case the luminescent species relaxes from its lowest excited triplet state by emitting a photon in a process called phosphorescence.

Time-Correlated Single Photon Counting (TCSPC) is a popular method for carrying out TRPL measurements. TCSPC works by measuring the time between sample excitation by a laser pulse and the arrival of the emitted photon at the detector. TCSPC requires a defined “start”, provided by the electronics steering the laser pulse or a photo diode, and a defined “stop” signal, realized by detection with single-photon sensitive detectors. The measurement of this time delay is repeated many times to account for the statistical nature of the fluorophores emission. The detected events are then sorted into a histogram according to their arrival time which allows reconstruction of the photoluminescence decay.

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Various types of set-up can be used for TRPL measurements, they can be broadly divided into three categories: spectrometers, confocal microscopes or combinations of both instruments.

For time-resolved experiments, the sample is excited by a pulsed laser, LED, or Xe-flash lamp. The emitted emission is detected by a detector with sensitivity in the UV/Vis or NIR spectral region.

Selection of the emission wavelength can be done via either a monochromator, long pass, band pass or variable filters. For lifetime measurements, either Time-Correlated Single Photon Counting (TCSPC) or Multi-Channel Scaling (MCS) electronics are used for data acquisition.

In summary, the essential components of a spectrometer set-up are:

  • pulsed source (flash lamp, pulsed laser, LEDs)
  • a way of separating fluorescence signal from excitation light (via monochromator or optical filters)
  • single photon sensitive detector in the UV/Vis or NIR region
  • for lifetime measurements: TCSPC or MCS unit to measure lifetimes from ps to ms

Coupling the time-resolved spectrometer with the scanning microscope. This combination allows scanning and recording data from any sample mounted on the microscope stage.

PicoQuant offers several systems that can be used for TRPL:

FluoTime 250 FluoTime 250

Compact and Modular Fluorescence Lifetime Spectrometer

The FluoTime 250 integrates all essential optics and electronics for time-resolved luminescence spectroscopy in a compact, fully automated device. This spectrometer is designed to assist the user in carrying out routine as well as complex measurements easily and with high reliability thanks to fully automated hardware components and a versatile system software featuring wizards providing step-by-step guidance.


FluoTime 300FluoTime 300

High Performance Fluorescence Lifetime Spectrometer

The FluoTime 300 "EasyTau" is a fully automated, high performance photoluminescence lifetime spectrometer with steady-state and phosphorescence option. The system features an ultimate sensitivity with a record breaking 24.000:1 Water Raman SNR. The FluoTime 300 contains the complete optics and electronics for recording TRPL decays by means of Time-Correlated Single Photon Counting (TCSPC) or Multichannel Scaling (MCS). The system is designed to be used with picosecond pulsed diode lasers, LEDs or Xenon lamps. Multiple detector options enable a large range of system configurations. The FluoTime 300 can be used to study photoluminescence between a few picoseconds and several seconds


MicroTime 100 - upright time-resolved confocal microscopeMicroTime 100

Upright time-resolved confocal microscope

The MicroTime 100 is an idea tool for the study of time-resolved photoluminescence of solid samples such as wafers, semiconductors or solar cells. It can also be used for mapping purposes or to measure intensity dependent TRPL. The system is based on a conventional upright microscope body that permits easy access to a wide range of sample shapes and sizes. The MicroTime 100 can be supplied with either manual scanning or with a 2D piezo scanner with either µm or cm resolution.


MicroTime 200 - inverse time-resolved confocal microscopeMicroTime 200

Inverse time-resolved confocal microscope

The MicroTime 200 time-resolved fluorescence microscope system is a powerful instrument capable of Fluorecence Corelation Spectroscopy and its daughter techniques as well as TRPL Imaging / Fluorescence Lifetime Imaging (FLIM) with single molecule detection sensitivity. It contains the complete optics and electronics for recording virtually all aspects of the fluorescence dynamics of microscopic samples or femtoliter volumes. The instrument gains its exceptional sensitivity and flexibility in combination with unprecedented ease-of-use from a unique fusion of miniaturized and highly sophisticated state-of-the-art technologies. Although, these technologies enable to run an instrument of comparable complexity and power without having to spend more time on instrument maintenance than on original scientific content, the MicroTime200 remains an open platform that allows the advanced scientist to easily built upon the open character of the instrument in order to realize highly customized applications.

The following core components are needed to build a system capable of fluorescence upconversion measurements, which are (partly) available from PicoQuant:

Charge carrier dynamics in semiconductors

Intensity dependent decay studies of GaAs-based quantum wells.

Charge carrier dynamics in semiconductors are determined by the architecture and function of the respective device and directly relate to the nature and quality of wafer materials. Therefore characterizing these materials require precise and efficient measurement techniques to determine the free charge carrier lifetime. For particular classes of semiconductors, the characteristic charge carrier lifetime is highly dependent on the nature and dimensions of the materials and interfaces involved. Furthermore, surface effects, passivation, and energy transfer efficiency of sensitizers as well as the presence of dopants, impurities and defect sites can introduce significant variations in this parameter. Since the photoluminescence of semiconductors is a directly related to the charge carrier dynamics, time-resolved photo-luminescence (TRPL) using time-correlatedsingle photon counting (TCSPC) is highly suited for their analysis and characterization, down to the sub-nanosecond time scale.Lifetime ranges of different materials: GaAs top, Si nanodots, bottom

Set-up of example above:

Sample courtesy of Andrea Knigge, Ferdinand-Braun-Institut, Berlin, Germany

Set-up of example below:

GaAs sample courtesy of Andrea Knigge, Ferdinand-Braun-Institut, Berlin, Germany
Si-nanodot sample courtesy of Maurizio Roczen, Helmholtz-Zentrum Berlin für Materialien und Energie, Germany


Determining electron-hole diffusion lengths in perovskite solar cells

Determining electron-hole diffusion lengths in perovskite solar cellsA critical parameter in understanding the photophysics of semiconductor solar cells is the diffusion length of the photo-excited electrons and holes. Time-resolved photoluminescence quenching experiments are a valuable tool for determining these diffusion lengths. The example shows data obtained from mixed halide and triiodide organometal perovskite layers in presence of either an electron (blue) or hole (red) quenching layer, and a PMMA coating (black). The decay curves were recorded at 780 nm, corresponding to the peak emission of both materials. The measured decay dynamics can be fitted to a specific diffusion model, allowing deriving diffusion lengths. Here, the diffusion length of the electrons and holes in the mixed halide perovskite was 1 μm while the triiodide material featured a much shorter length of 100 nm, correlating well with performance of these materials as solar cells.

Set-up:

Reference: S. D. Stranks et al., Science, 342 (2013), p.341


Label-free detection of native analyte fluorescence performed on a fluidic microchip

Fluorescence decaysTime-resolved fluorescence microscopy in the deep UV can be employed in microfluidic environments to enable label-free detection and identification of various aromatic analytes in chip electrophoresis. The results show the electrophoretic separation of a mixture of small aromatics (188 µM Serotonin, 135 µM propranolol, 238 µM 3-phenoxy-1.2-propanediol and 196 µM tryptophan). Fluorescence decay curves are gathered on-the-fly and average lifetimes can be determined for different substances in the electropherogram to identify the aromatic compounds in the mixtures. Based on Time-Correlated Single Photon Counting, the background fluorescence can be discriminated which results in an improved signal-to-noise ratio. Additionally, microchip electrophoretic separations with fluorescence lifetime detection can be carried out for protein mixtures, which can be very useful for biopolymer analysis.

Set-up:

  • MicroTime 200
  • Excitation: 266 nm
  • Detection: bandpass 350/50, DUG11x (285-365 nm), super bialkali PMT
  • Analysis: SymPhoTime

Decay associated spectra of tryptophan

Decay associated spectra - tryptophanA solution of L-tryptophane in sline buffer was excited at 290 nm. A series of time-resolved emission spectra was collected in the wavelength range from 310 to 460 nm with a step size of 5 nm. Time-resolved emission spectroscopy (TRES) can identify different species of a mixture, even with strongly overlapping spectra, if their lifetimes are different. With modern TCSPC electronics and fast excitation sources one can get spectral as well as kinetic information within minutes.

Set-up:


Thermally-Activated Delayed Fluorescence (TADF) of acryflavine in PVA films

Thermally-activated delayed fluorescenceAcryflavine embedded in a PVA matrix exhibits both normal fluorescence with ns lifetime as well as thermally activated delayed fluorescence with lifetimes in ms time range. Upon excitation to its first excited singlet state S1, acryflavine undergoes efficient intersystem crossing (ISC) to the first excited triplet state T1. Since the energy gap between T1 and S1 states is small, there is a probability of thermally activated back ISC T1 → S1 followed by emission of TADF with lifetime of the T1 state, but at the same wavelength as normal fluorescence (S1 → S0 transition). When acryflavine is dissolved in water, the T1 state is efficiently quenched so that only the fast fluorescence can be observed.

With a single setup (FT300 equiped with TH260 TCSPC electronics and PDL820 laser driver) one can efficiently measure lifetimes in both ns as well as ms time ranges.

Set-up:

  • FluoTime 300
  • Excitation at 440 nm using a LDH-P-C-440 operated in pulsed as well as in burst mode using a PDL 820
  • Analysis: EasyTau 2