header image materials science

Materials Science

Quantum Dots and Nanoparticles

Investigation of Nano Materials

Quantum dots (QD) are semiconductor particles with sizes of a few nm. QD emit light of a specific wavelength when a current is applied or exposed to light. The emission wavelength can be tuned by changing either the size, shape, material, or by doping the QDs. Smaller QDs (2–3 nm) emit light at short wavelengths (blue-green spectral region), while larger QDs (5–6 nm) will emit light in the longer wavelengths (orange, red, or IR). Furthermore, it has been shown that their fluorescence lifetime is also tied to particle size. In larger dots, the lifetime is longer due to more closely spaced energy levels in which the electron-hole pair can be trapped.

Nanoparticles (NPs) are also very small structures but larger than QDs, usually ranging from 8 to 100 nanometers. Because of this, NPs exhibit behaviors between those bulk materials and atoms or molecules. NPs often possess unexpected optical properties as their size allows for quantum confinement effects. Additionally, the interfacial layers surrounding NPs play an important role in all of their physical properties. These layers typically consists of ions, inorganic material, or organic molecules.

By controlling their size, shape as well as composition, the absorption properties of NPs can be fine-tuned to fit the needs of photovoltaic or solar thermal applications. QDs are also of great interest for display or lighting applications where their stability and tunable emission properties are very desirable.

Both time-resolved as well as steady-state luminescence spectroscopy are excellent tools for investigating the excited state characteristics and dynamics of both NPs and QDs.

Typical application areas for such materials are:

  • LEDs
  • Solar cell
  • Diode lasers and second-harmonic generation
  • Displays
  • Photodetectors
  • Photocatalysts
  • Transistors
  • Quantum computing
  • Medical applications as imaging markers, tumors detection or photodynamic therapy

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


Scheme of the general layout of a fluorescence spectrometer Scheme of the general layout of a fluorescence scanning microscope Coupling theFluoTime 300  time-resolved spectrometer with the MicroTime 100 scanning microscope. This combination allows scanning and recording data from any sample mounted on the microscope stage.

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


FluoTime 300 - fully automated high performance fluorescence spectrometerFluoTime 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 fluorescence 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..

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

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

Transient TRPL spectrum of a GaAsP quantum well systemTransient 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.


Photoluminescence spectra of a GaAsP quantum well systemThis 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.


Intensity dependent photoluminescence decays of a GaAsP quantum wellExcitation 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.


  • 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

Latest 10 publications related to Time-resolved Fluorescence

The following list is an extract of 10 recent publications from our bibliography that either bear reference or are releated to this application and our products in some way. Do you miss your publication? If yes, we will be happy to include it in our bibliography. Please send an e-mail to info@picoquant.com containing the appropriate citation. Thank you very much in advance for your kind co-operation.