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Quantum dots - Size determination by SAXS

A dispersion of nanosized CdSe quantum dots was analyzed with the SAXSpace system. The particle size, which is a very crucial parameter for the properties of quantum dots, was determined within a very short time.

Introduction

Quantum dots (QDs) are semiconducting materials whose properties and characteristics are strongly related to their size. QDs are used in various applications like LED technology and biological imaging techniques. In comparison to e.g. traditional organic fluorescent dyes, the colour of quantum dots is a lot more stable and much brighter.


 
The precise determination of size and size distribution of quantum dots is very important as the size determines their properties like fluorescence, conductivity and others. For example, after excitation small quantum dots emit light with a smaller wavelength (blue shift) compared to larger quantum dots (red shift). Ideally all particles in a sample should have a narrow size distribution ensuring a precise, i.e. monochromatic wavelength of the emitted light.
Therefore, SAXS is a helpful method to control the properties of quantum dots because it quickly determines the particle size and produces representative results.

Experimental and Results

CdSe quantum dots, dispersed in heptane, were analyzed with the SAXSpace system (measurement time: 1 minute). The background-subtracted scattering curve (Figure 1) shows pronounced minima indicating high monodispersity.

Figure 1: Background-subtracted scattering curve

The size distribution (Figure 2) was calculated with program GIFT1.
Figure 2: Size distribution function

The CdSe quantum dot particles are highly monodisperse with an average radius of approx. 1.7 nm.


1    A. Bergmann, G. Fritz, O. Glatter, J. Appl. Cryst. 33 (2000) 1212

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