What are Quantum Dots?


Introduction

Over the past few years, fluorescent semiconductor nanoparticles have gained a lot of importance. In comparison with traditional fluorescent dyes, they show considerable benefits. These nanocrystals comprise an inorganic core and the composition and size of the inorganic core decide their optical characteristics (Figure 1).

Figure 1. Quantum dots

The core features excellent fluorescence properties as it exhibits high stability against environmental conditions such as irradiation, air or temperature. The stabilization of organic molecules surrounding this inorganic core enables solubility in organic solvents such as toluene, hexanes or chloroform. It is possible to obtain dispersibility in aqueous media without losing the quantum dot’s original properties by interchanging these organic ligands in the outer sphere via water soluble molecules.

Light absorption used for excitation in the case of nanoparticles is normally larger when compared to fluorescent dyes. Hence the detection of these particles can be done at very low concentrations and even a single particle could be investigated using spectroscopic methods.

Semiconductor Nanoparticles

Semiconductor nanoparticles are the most studied system among fluorescent particles. By modifying the particle size, the band gap of these systems and therefore the emission wavelength can be manipulated. Hence these systems are highly attractive. The smaller the quantum dot size, the bigger is the band-gap and shorter is the emitted light wavelength. This effect is termed as size quantization.

By taking a collective decision on material size and composition, it is possible to cover the complete visible light spectrum up to the IR region. Using this concept, CAN has developed the CANdots Series A, B and C.

CANdots Series

The CANdots series from CAN covers the following:

  • Series A covers the spectrum’s visible region having emission wavelengths from 500 to 650nm
  • Series B covers the spectrum’s near IR section with emission wavelengths from 650 to 800nm
  • Series C covers the spectrum’s IR region with emission maxima more than 1000nm

As the absorption increases from the emission maximum while moving towards shorter wavelengths, the nanoparticles can be excited with all wavelengths below the emission. In comparison with organic fluorescent dyes, it is not necessary to reset the excitation wavelength for each dye. A complete set of a variety of nanoparticles can be excited all together with a single excitation wavelength and their emission can be detected.

After excitation, i.e. after light absorption, an electron-hole pair is generated in semiconductor nanocrystals. This exciton or electron-hole pair is free to move within the core until recombination or emission of light occurs. During this time (typically around 10-20ns), the charge carriers may be bound at some other place and consequently there may be a decrease in emission intensity.

In order to prevent this, the semiconductor nanoparticles or cores were enclosed by passivating inorganic shells. The particle stability is improved with these shells. In addition, there is an improvement in the quantum yield of the system such as in CAN Series A core/shell (CdSe/CdS) and core/shell/shell (CdSe/ZnSe/ZnS) systems.

About Center for Applied Nanotechnology (CAN) GmbH

The Center for Applied Nanotechnology (CAN) GmbH offer companies and other institutions bilateral contract R&D services in the area of nanotechnology.

Furthermore, CAN GmbH participate in national and international research programs. They focus on the utilization of new concepts in nanochemistry, especially in the fields of energy (components for solar and fuel cells), life sciences (diagnostic agents) and home & personal care (cosmetics, detergents, specialty polymers) and corresponding nanoanalysis.

Their main expertise is the production of various nanoscaled materials like fluorescent, magnetic and catalytically-active nanocrystals. Since 2005 they are producing a variety of nanoparticles with different properties: quantum dot materials with fluorescent features (Series A in the visible and Series C in the NIR/IR-range), rare-earth doped nanoparticles (Series X – blue, green, red), magnetic particles (Series M – iron oxide) and plasmonic gold nanoparticles (Series G).

These products are marketed under the brand CANdots® and are dispersible in polar or unpolar media readily available for applications in research and industry.

This information has been sourced, reviewed and adapted from materials provided by Center for Applied Nanotechnology (CAN) GmbH.

For more information on this source, please visit Center for Applied Nanotechnology (CAN) GmbH.

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