Determining Quantum Dot Size Using UV-Vis Spectrometry

Quantum dots (QDs) are semiconducting nanoparticles with distinct optical and electrical properties that fall between bulk materials and discrete molecules.

• QDs' size-dependent tunability makes them suitable for several applications, including photovoltaics, biomedical imaging, and display technology.
• The DB30 UV-Vis Spectrophotometer accurately measures QD diameters in accordance with ISO/TS 17466.

These distinguishing characteristics are attributable in part to QDs' unusually high surface-to-volume ratio, which means that a considerable proportion of their atoms are placed on the surface of the crystalline lattice.

In addition, the radii of QDs typically range from 2 to 10 nm, resulting in strong quantum confinement effects.

These effects split the electronic bands in QDs into separate energy levels, altering the material's properties.

As the size of a QD decreases, the band gap widens (Figure 1), causing a shift in the wavelength of emitted light, electrical conductivity, and optical characteristics of the QD.

QDs' size-dependent tunability makes them valuable in a variety of applications, including photovoltaics, biomedical imaging, and display technologies.

Figure 1. Band structure of a bulk semiconductor vs. QDs, and effects of QD size on band gap. Arrows represent the transition corresponding to the first excitonic absorption. Image Credit: Edinburgh Instruments

In this article, the Edinburgh Analytical DB30 UV-Vis Spectrophotometer (Figure 2) is used to characterize CdSe/ZnS QD samples in solution.

ISO/TS 17466 (Use of UV/Vis absorption spectroscopy in the evaluation of cadmium chalcogenide colloidal quantum dots, 2015) defines a direct relationship between the first excitonic absorption peak and particle size for determining the diameter of QDs.¹

The DB30's touchscreen interface, high-scanning rate, and double-beam technique provide simple, speedy QD analysis.

Figure 2. Edinburgh Analytical DB30 UV-Vis Spectrophotometer. Image Credit: Edinburgh Instruments

Sample Preparation

CdSe/ZnS QDs were dissolved in toluene (Sigma Aldrich) to the desired concentration (absorbance < 1 at the first excitonic absorption peak) and inserted into 10 mm pathlength quartz cuvettes. Toluene was used as a reference.

Instrument Configuration

UV-Vis spectroscopic measurements were taken with an Edinburgh Analytical DB30 UV-Vis Spectrophotometer, and settings were selected using the touchscreen interface (Figure 3). A narrow wavelength range was used to better locate the first excitonic absorption peak.

Figure 3. Experimental parameters on the DB30 UV-Vis Spectrophotometer touchscreen interface. Image Credit: Edinburgh Instruments

Procedure

1. A baseline measurement was conducted with an empty sample chamber.
2. A cuvette with the QD solution was inserted into the sample beam location, and the toluene reference cuvette was added to the reference beam position.
3. A scan was performed using the specified settings (Figure 3).
4. The initial excitonic absorption peak was found by determining the longest wavelength local maximum in the absorption spectra.
5. The diameter of the QDs in the sample was calculated using an empirical size calculation from ISO/TS 17466 based on their wavelength.¹

Diameter of CdSe QDs from UV-Vis Absorption Spectra

The first excitonic absorption peak corresponds to the lowest-energy optical transition between the QD's discrete electronic states (Figure 1).

DB30's Peaks and Troughs analysis revealed a peak for the produced CdSe/ZnS solutions at λ = 526.0 nm (Figure 4). It was explained by the excitonic transition from the ground state in the valence band to the first excitonic excited state in the conduction band.

Figure 4. UV-Vis absorption spectrum of CdSe QDs taken from the DB30 touchscreen interface. The Peaks and Troughs function determined the first excitonic absorption peak at 526.0 nm. Image Credit: Edinburgh Instruments

The link between a QD's diameter (D) and its initial excitonic absorption peak is empirically measured using transmission electron microscopy (TEM) measurements, or X-ray diffraction (XRD) in the case of very small CdSe nanoclusters ²

In this case, the empirical sizing equation was derived from ISO/TS 17466, while some scholarly articles specify different coefficients based on likely TEM measurement errors.^1,3

The relationship formula for ISO/TS 17466 is as follows:

D (nm) = 41.57 – 0.4277λ + 1.6242 x 10^-3λ² – 2.6575 x 10^-6λ³ + 1.6122 x 10^-9λ⁴

The produced solution contained CdSe/ZnS QDs with a diameter of D = 2.64 nm. This demonstrated that UV-Vis absorption spectroscopy on the DB30 UV-Vis Spectrophotometer is a quick way to estimate QD diameter.

Conclusion

This article explained how to use the Edinburgh Analytical DB30 UV-Vis Spectrophotometer in a spectroscopic approach to determine the diameter of CdSe/ZnS QDs.

Given the growing range of biomedical and technological applications for QDs, a straightforward approach to estimating their size is becoming increasingly vital. The DB30's user-friendly touchscreen interface and fast scan speed enable quick and easy QD analysis and characterization.

References

1. ISO (2015). ISO/TS 17466: Use of UV-Vis absorption spectroscopy in the characterization of cadmium chalcogenide colloidal quantum dots. Available at: https://www.iso.org/standard/59853.html.
2. Yu, W.W., Qu, L., Guo, W. and Peng, X. (2003). Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. Chemistry of Materials, 15(14), pp.2854–2860. DOI: 10.1021/cm034081k. https://pubs.acs.org/doi/10.1021/cm034081k.
3. Jasieniak, J., et al. (2009). Re-examination of the Size-Dependent Absorption Properties of CdSe Quantum Dots. The Journal of Physical Chemistry C, 113(45), pp.19468–19474. DOI: 10.1021/jp906827m. https://pubs.acs.org/doi/10.1021/jp906827m.

This information has been sourced, reviewed, and adapted from materials provided by Edinburgh Instruments.

For more information on this source, please visit Edinburgh Instruments.