13 Controlling Optical Properties of Multinary Quantum Dots …
235
power of ZAITe QDs was variable depending on the irradiated photon energy. This
behavior could be reasonably explained by the hot hole transfer from ZAITe QDs into
ITO electrodes as schematically illustrated in Fig. 13.9b, the probability of which
was increased for hot holes at more highly excited states.
The difference between the onset potential and the E VB was at most ca. 0.37 V as
shown in Fig. 13.9c, being much smaller than that between the E g of ZAITe QDs used
and the excitation photon energy. This indicated that a large part of the excess energy
of hot holes was dissipated as heat before the hole transfer to collecting electrodes.
Furthermore, the onset potential shift with irradiation of 3.40-eV photons became
larger with a decrease in the E g of ZAITe QDs, suggesting that ZAITe QDs having
a lower E CB (Fig. 13.6) could produce hot holes at more highly excited states. It
should be noted that a shift of the photocurrent onset potential was not observed
for ZAITe QD multilayer films prepared with 5 deposition cycles, regardless of the
particle composition: The onset potential agreed well with the E VB of ZAITe QDs,
indicating that hot holes were relaxed to the valence band bottom during the transfer
between QDs in multilayer films.
13.5 Conclusion
We described solution-phase preparation of multinary QDs composed of less-toxic
Ag-III-VI-based semiconductors, enabling precise control of their chemical composition, size, shape, and heterostructure. Multinary QDs had E g s that are tunable by
both the chemical composition and particle size. Controlling photoluminescence
properties of multinary QDs by the chemical composition is a significant advantage
in comparison with conventional binary QDs. A band-edge PL peak was successfully
observed by tuning the non-stoichiometric composition of Ag-In-Ga-S QD cores and
by surface coating with a GaS x shell layer, the peak width of which was much smaller
than that of a defect-site emission peak. Irradiation to Zn-Ag-In-S QDs enabled
photocatalytic H 2 evolution, the activity being modulated by the particle composition and morphology. A nonlinear photoelectrochemical response was observed by
visible light irradiation to near-IR-light-responsive QDs composed of a Zn-Ag-In-Te
solid solution, in which a cathodic photocurrent was generated at a potential more
positive than the E VB of QDs. Further precise control of the photochemical properties
will be achieved by reducing the inhomogeneity in both the chemical composition
and the particle size of individual multinary QDs. Our findings and the underlying
mechanism will be essential for the development of novel QD-based devices such as
bio-imaging devices, luminescent devices, photocatalysts, and solar cells.
Acknowledgements The present work was supported by JSPS KAKENHI Grant Numbers
JP15H01082 and JP17H05254 in Scientific Research on Innovative Areas “Photosynergetics” and
JP16H06507 in Scientific Research on Innovative Areas “Nano-Material Optical-Manipulation.”
235
power of ZAITe QDs was variable depending on the irradiated photon energy. This
behavior could be reasonably explained by the hot hole transfer from ZAITe QDs into
ITO electrodes as schematically illustrated in Fig. 13.9b, the probability of which
was increased for hot holes at more highly excited states.
The difference between the onset potential and the E VB was at most ca. 0.37 V as
shown in Fig. 13.9c, being much smaller than that between the E g of ZAITe QDs used
and the excitation photon energy. This indicated that a large part of the excess energy
of hot holes was dissipated as heat before the hole transfer to collecting electrodes.
Furthermore, the onset potential shift with irradiation of 3.40-eV photons became
larger with a decrease in the E g of ZAITe QDs, suggesting that ZAITe QDs having
a lower E CB (Fig. 13.6) could produce hot holes at more highly excited states. It
should be noted that a shift of the photocurrent onset potential was not observed
for ZAITe QD multilayer films prepared with 5 deposition cycles, regardless of the
particle composition: The onset potential agreed well with the E VB of ZAITe QDs,
indicating that hot holes were relaxed to the valence band bottom during the transfer
between QDs in multilayer films.
13.5 Conclusion
We described solution-phase preparation of multinary QDs composed of less-toxic
Ag-III-VI-based semiconductors, enabling precise control of their chemical composition, size, shape, and heterostructure. Multinary QDs had E g s that are tunable by
both the chemical composition and particle size. Controlling photoluminescence
properties of multinary QDs by the chemical composition is a significant advantage
in comparison with conventional binary QDs. A band-edge PL peak was successfully
observed by tuning the non-stoichiometric composition of Ag-In-Ga-S QD cores and
by surface coating with a GaS x shell layer, the peak width of which was much smaller
than that of a defect-site emission peak. Irradiation to Zn-Ag-In-S QDs enabled
photocatalytic H 2 evolution, the activity being modulated by the particle composition and morphology. A nonlinear photoelectrochemical response was observed by
visible light irradiation to near-IR-light-responsive QDs composed of a Zn-Ag-In-Te
solid solution, in which a cathodic photocurrent was generated at a potential more
positive than the E VB of QDs. Further precise control of the photochemical properties
will be achieved by reducing the inhomogeneity in both the chemical composition
and the particle size of individual multinary QDs. Our findings and the underlying
mechanism will be essential for the development of novel QD-based devices such as
bio-imaging devices, luminescent devices, photocatalysts, and solar cells.
Acknowledgements The present work was supported by JSPS KAKENHI Grant Numbers
JP15H01082 and JP17H05254 in Scientific Research on Innovative Areas “Photosynergetics” and
JP16H06507 in Scientific Research on Innovative Areas “Nano-Material Optical-Manipulation.”