Quantum Dots and Nanoparticles

The photophysical properties of QDs or NPs in nanostructures, films or devices can be investigated using spectrometers, microscopes or a combination of both instruments.

In time-resolved experiments, the sample is excited by a pulsed laser, LED or Xe-flash lamp, while a Xe lamp or a CW laser are used for excitation in steady-state experiments. Light emitted by the QDs/NPs is detected using a detector with sensitivity in UV/Vis or NIR region after passing through monochromator or bandpass filter for wavelength selection.

Data for lifetime measurements is acquired with either Time-Correlated Single Photon Counting (TCSPC) or Multi-Channel Scaling (MCS) electronics.

In summary, the essential components for such a set-up:

• pulsed or CW excitation source
• monochromator or filters for wavelength selection
• single photon sensitive detector in the UV/Vis and/or in the NIR spectral range
• for steady-state measurements: MCS unit
• for lifetime measurements: TCSPC or MCS unit to measure lifetimes ranging from ps to ms

PicoQuant offers the following system that can characterize quantum dots and nanoparticles:

FluoTime 300

Fully Automated High Performance Fluorescence Lifetime Spectrometer

The FluoTime 300 "EasyTau" is a fully automated, high performance fluorescence lifetime spectrometer with steady-state and phosphorescence option. It contains the complete optics and electronics for recording fluorescence 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. With the FluoTime 300 decay times down to a few picoseconds can be resolved.

MicroTime 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..

The following core components are needed to build a system capable of studying nanoparticles or quantum dots. These components are partly available from PicoQuant:

• Laser drivers
  • PDL 800-D
  • Taiko PDL M1
  • PDL 828 "Sepia II"
• Laser or LED heads
  • LDH Series
  • LDH-FA Series
  • PLS Series
• Photon Counting Detector
  • UV/Vis sensitive PMA Hybrid Series
  • UV/Vis sensitive Photomultiplier Detector Assembly
  • NIR sensitive NIR Photomultiplier Detector
• TCSPC and MCS Electronics
  • PicoHarp 300
  • TimeHarp 260 PICO (with long range mode)
  • TimeHarp 260 NANO
• Analysis software
  • EasyTau 2
  • SymPhoTime 64

Using time-resolved photoluminescence (TRPL) to sudy a GaAsP quantum well system

Transient TRPL spectrum of a quantum well structure illuminated at 595 nm and measured with a fluorescence lifetime spectrometer showing (a) the layer structure of the quantum well and (b) the time-resolved emission spectrum (TRES) from the wafer. The emission peak at 650 nm stems from the Al0.4Ga0.6As-barrier, the band around 735 nm from the GaAsP quantum well and the peak at around 860 nm from the n-GaAs layer and the GaAs substrate. The decays recorded for each spectral channel can be well described with a three-component exponential model. Only the average lifetime and longest component of the fits are displayed. The measurement exemplifies the correlation of characteristic charge carrier dynamics in material specific spectral channels of the multi-component system.

Set-up:

• FluoTime 300
• Excitation at 595 nm using a LDH-P-FA-595 using a PDL 800-D
• Analysis: EasyTau 2

This figure shows excitation spectra that were recorded at three different wavelengths: one at 650 nm corresponding to the peak of the Al0.4Ga0.6As-barrier (blue); the second at 735 nm which is due to the quantum well layer (light green); the third at 860 nm that is associated to the n-GaAs-layer and GaAs substrate (dark green). The spectrum of the quantum well layer shows a prominent drop in intensity around 650 nm indicating an interaction with the barrier layer. The n-GaAs-layer and GaAs substrate on the other hand show an increase in intensity around 650 nm, which correlates with the absorption edge in the barrier at wavelengths longer than the barrier band gap. The rectangles illustrate the band gaps of the corresponding layers.

Set-up:

• FluoTime 300
• Excitation: CW Xe lamp
• Detection: 650, 735, and 860 nm
• Analysis: EasyTau 2

Excitation intensity dependent time-resolved photoluminescence of the GaAsP quantum well. A 635 nm laser was focused onto a 100 µm spot (FWHM) and the emission was passed through a a 665 nm longpass filter. The inset shows a saturation effect in the average lifetime for increasing excitation intensity. The average lifetime approaches a fixed value, as expected for high injection conditions in Shockley-Read-Hall determined photoluminescence.

Set-up:

• MicroTime 100
• Excitation: 635 nm with LDH-P-C-635B using a PDL 828
• Detection: PDM 1CTC-SPAD-Detector, detection wavelength > 664 nm

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