522
M. Vacha and S. Hirata
This behavior was reversible and the initial state recovered after removal of the
electric field, as shown in Fig. 30.3c. Overall, about 45% of the CsPbBr 3 NCs in
the PMMA matrix under the influence of the applied electric field show blinking
dynamics which is similar to that of CsPbBr 3 in PVK under no electric field. This
result might reflect electric field-enhanced charging–discharging of CsPbBr 3 NCs in
PMMA. To summarize, externally applied electric field modifies the PL intensity of
single CsPbBr 3 NCs in a similar way as the presence of a conductive matrix.
The electric field can potentially modulate also the PL energy, causing the socalled Stark shift. The dependence of a PL spectrum on continuously applied DC
electric field within the range of −1.0 and +1 MV/cm is plotted on the top of
Fig. 30.3d in the form of a spectral image. PL spectra for individual applied voltage
values were analyzed by Gaussian fitting, and the peaks are plotted in the bottom of
Fig. 30.3d. The observed PL peak energy changes from 2.405 to 2.430 eV within
the applied voltage range, and the effect fully reversible. As also seen in Fig. 30.3d,
removal of the electric field causes the PL peak to reverse to energy observed at
0 MV/cm during the sweep. The recovery of the PL peak position shows that the
observed phenomenon is a reversible effect due to the electric field and is not affected
by the degradation of CsPbBr 3 NCs during the acquisition of the data. This fielddependent PL emission energy confirms the presence of a DC field-induced, nonlinear
Stark effect. Fitting of the experimental data to a combination of a linear and quadratic
equations (Fig. 30.3d) indicates both polar and polarizable characters of the electric
field-induced phenomenon. Appearance of a linear component of the Stark effect in
inorganic halides nanocrystals such as CsPbX 3 is generally not expected because of
symmetry considerations and the presumed lack of a permanent dipole. However,
interactions between CsPbBr 3 NCs and the PMMA matrix may cause anisotropic
distributions of charges in CsPbBr 3 NCs, resulting in a polar character.
30.2.3 Single-Particle Spectroscopy of I-III-VI
Semiconductor Nanocrystals: Spectral Diffusion
and Suppression of Blinking
Nanocrystals of ternary I-III-VI semiconductors with a direct bandgap are promising
materials for various applications due to their excellent optoelectronic properties
[23]. Tunable band gap, large extinction coefficient, and low toxicity make them
potential alternatives to II–VI semiconductor quantum dots (QD). However, large
photoluminescence spectral width and moderate brightness have long restricted their
practical use. While it has been recently shown that the large bandwidth can be
overcome by capping with an appropriate shell, detailed knowledge of the physical
origin of the spectral broadening of the defect-related emission is indispensable
for further material development. Here we use single-particle PL spectroscopy on
nanocrystals of the composition (AgIn) x Zn 2(1−x) S 2 to study the origin of the spectral
broadening and PL blinking [16].
M. Vacha and S. Hirata
This behavior was reversible and the initial state recovered after removal of the
electric field, as shown in Fig. 30.3c. Overall, about 45% of the CsPbBr 3 NCs in
the PMMA matrix under the influence of the applied electric field show blinking
dynamics which is similar to that of CsPbBr 3 in PVK under no electric field. This
result might reflect electric field-enhanced charging–discharging of CsPbBr 3 NCs in
PMMA. To summarize, externally applied electric field modifies the PL intensity of
single CsPbBr 3 NCs in a similar way as the presence of a conductive matrix.
The electric field can potentially modulate also the PL energy, causing the socalled Stark shift. The dependence of a PL spectrum on continuously applied DC
electric field within the range of −1.0 and +1 MV/cm is plotted on the top of
Fig. 30.3d in the form of a spectral image. PL spectra for individual applied voltage
values were analyzed by Gaussian fitting, and the peaks are plotted in the bottom of
Fig. 30.3d. The observed PL peak energy changes from 2.405 to 2.430 eV within
the applied voltage range, and the effect fully reversible. As also seen in Fig. 30.3d,
removal of the electric field causes the PL peak to reverse to energy observed at
0 MV/cm during the sweep. The recovery of the PL peak position shows that the
observed phenomenon is a reversible effect due to the electric field and is not affected
by the degradation of CsPbBr 3 NCs during the acquisition of the data. This fielddependent PL emission energy confirms the presence of a DC field-induced, nonlinear
Stark effect. Fitting of the experimental data to a combination of a linear and quadratic
equations (Fig. 30.3d) indicates both polar and polarizable characters of the electric
field-induced phenomenon. Appearance of a linear component of the Stark effect in
inorganic halides nanocrystals such as CsPbX 3 is generally not expected because of
symmetry considerations and the presumed lack of a permanent dipole. However,
interactions between CsPbBr 3 NCs and the PMMA matrix may cause anisotropic
distributions of charges in CsPbBr 3 NCs, resulting in a polar character.
30.2.3 Single-Particle Spectroscopy of I-III-VI
Semiconductor Nanocrystals: Spectral Diffusion
and Suppression of Blinking
Nanocrystals of ternary I-III-VI semiconductors with a direct bandgap are promising
materials for various applications due to their excellent optoelectronic properties
[23]. Tunable band gap, large extinction coefficient, and low toxicity make them
potential alternatives to II–VI semiconductor quantum dots (QD). However, large
photoluminescence spectral width and moderate brightness have long restricted their
practical use. While it has been recently shown that the large bandwidth can be
overcome by capping with an appropriate shell, detailed knowledge of the physical
origin of the spectral broadening of the defect-related emission is indispensable
for further material development. Here we use single-particle PL spectroscopy on
nanocrystals of the composition (AgIn) x Zn 2(1−x) S 2 to study the origin of the spectral
broadening and PL blinking [16].