Emission Tail of Indium Phosphide Quantum Dots

Semiconductor quantum dots (QDs) have unique tunable photoluminescence characteristics which make them ideal for a range of crucial technological applications including solid-state lighting, photovoltaics, displays, and biomedical imaging.

As a non-toxic and environmentally friendly alternative to traditional heavy metal-based QDs containing cadmium and lead, indium phosphide (InP) QDs have attracted significant interest. Due to non-radiative recombination occurring at trap states on the surface of the InP, QDs which are solely composed of InP are non-emissive.

In order to acquire emissive QDs, the InP core is coated with a layer of a higher bandgap semiconductor like zinc sulfide (ZnS) to form a core-shell heterostructure which passivates the trap states and heightens the photoluminescence quantum yield greatly, as seen in Figure 1.

Relationships between the composition and photoluminescence properties must be established to further enhance the stability, brightness, and color range of InP/ZnS QDs. In this article, the FS5 Spectrofluorometer is employed to perform a steady-state and time-resolved characterization of novel InP/ZnS QDs.

Structure and band energy diagram of core-shell InP/ZnS quantum dots.

Figure 1. Structure and band energy diagram of core-shell InP/ZnS quantum dots.

Materials and Methods

A solution of InP/ZnS QDs in toluene was prepared which had an absorbance of 0.15 at 500 nm. The absorption and photoluminescence properties were characterized using an FS5 Spectrofluorometer that was equipped with an SC-05 Cuvette Holder Module, time-correlated single-photon counting (TCSPC) electronics, an EPL-405 pulsed diode laser, and a PMT-980 detector.

FS5 Spectrofluorometer equipped with picosecond pulsed diode laser.

Figure 2. FS5 Spectrofluorometer equipped with picosecond pulsed diode laser.

Results and Discussion

The photoluminescence and absorption spectra are shown in Figure 3 and were quantified by utilizing the FS5. The FS5 contains an absorption detector as standard which permits the absorption and photoluminescence spectra to be measured quickly via a single instrument.

The spectra show that the InP/ZnS QDs have a pronounced band-edge photoluminescence peak at 620 nm with an FWHM of 65 nm. There is also a broad low energy tail extending out into the NIR, in addition to the primary band-edge peak. To detect this tail, the FS5 was equipped with an extended range photomultiplier tube detector (PMT-980) which has good sensitivity out to ~950 nm.

The PMT-980 is ideal for materials with long emission tails and supplies an extra 80 nm of detection range over the standard PMT-900. The broad low energy tail has been previously seen in InP/ZnS QDs and is a sign of trap emission.1

Absorption (black) and photoluminescence (red) spectra of the InP/ZnS QD solution. Absorption parameters: ??ex = 2 nm. Emission parameters: ?ex = 400 nm, ??ex = 8 nm, ??em = 3 nm.

Figure 3. Absorption (black) and photoluminescence (red) spectra of the InP/ZnS QD solution. Absorption parameters: Δλex = 2 nm. Emission parameters: λex = 400 nm, Δλex = 8 nm, Δλem = 3 nm.

The time response of the photoluminescence was measured by utilizing the TCSPC functionality of the FS5 to confirm the presence of traps; since trap emission and band-edge emission happen on different timescales. 

A 405 nm picosecond pulsed diode laser (EPL-405) was utilized to excite the sample, and the photoluminescence decay measured as a function of wavelength to build up the time-resolved emission spectrum (TRES) observed in Figure 4. The TRES map clearly indicates that, at longer emission wavelengths, the photoluminescence lifetime substantially heightens.

Time-resolved emission spectrum (TRES) of InP/ZnS QD solution measured using TCSPC. The photoluminescence intensity at each wavelength was normalised to 1. Experimental parameters: ?ex = 405 nm, ??em = 15 nm.

Figure 4. Time-resolved emission spectrum (TRES) of InP/ZnS QD solution measured using TCSPC. The photoluminescence intensity at each wavelength was normalized to 1. Experimental parameters: λex = 405 nm, Δλem = 15 nm.

In order to establish the photoluminescence lifetimes, the decay at the tail emission region (775 nm) and band-edge emission peak (620 nm) was measured with higher temporal resolution and 105 counts at the peak. The decays fit with four exponential components using the FS5’s Fluoracle® software, as seen in Figure 5.

Photoluminescence decays measured at (a) 620 nm and (b) 775 nm. The decays were measured using TCSPC and fit with four exponentials components and the intensity weighted average lifetime calculated. Experimental parameters: ?ex = 405 nm, ?em = 620 nm / 775nm, ??em = 10 nm.

Figure 5. Photoluminescence decays measured at (a) 620 nm and (b) 775 nm. The decays were measured using TCSPC and fit with four exponentials components and the intensity weighted average lifetime calculated. Experimental parameters: λex = 405 nm, λem = 620 nm / 775nm, Δλem = 10 nm.

Whilst the tail emission was found to have an average photoluminescence lifetime of 348 ns, the band-edge emission peak had a lifetime of 31.2 ns. The fact that tail region decay has an order of magnitude greater lifetime supplies more evidence that it occurs because of trapping within the InP/ZnS QDs.

In Figure 6, a potential mechanism is shown where the electrons at the conduction band edge become trapped at surface defects on the InP core that have energies within the bandgap. Slowly, these trapped electrons radiatively recombine with holes in the valence band, emitting light at a longer wavelength than the band-edge emission.

Schematic representation of the origin of band-edge emission and trap emission in QDs.

Figure 6. Schematic representation of the origin of band-edge emission and trap emission in QDs.

The presence of traps indicates that the ZnS shell could be incomplete and the InP is therefore not fully passivated. This means that the synthesis procedure should be modified in order to deposit a thicker ZnS shell.

Conclusion

A combined time-resolved and steady-state examination of the photophysics of InP/ZnS QDs was performed by utilizing the FS5 Spectrofluorometer. This revealed the presence of trap emission, which was potentially because of incomplete ZnS shelling.

This article highlights the capability of the FS5 Spectrofluorometer to characterize the emission, absorbance, and lifetime of novel quantum dot emitters in a single compact instrument and help to determine structure-property relationships.

References and Further Reading

  1. R. Toufanian, A. Piryatinski, A. H. Mahler, R. Iyer, J. A. Hollingsworth & Allison M. Dennis, Bandgap Engineering of Indium Phosphide-Based Core/Shell Heterostructures Through Shell Composition and Thickness, Front. Chem 6 567 (2018)

Acknowledgments

Produced from materials originally authored by Stuart Thomson from Edinburgh Instruments.

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

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

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