30 Single-Molecule Level Study and Control …
533
30 nm) to form the HDP. Triplet energy levels of AC and DPA are lower than that of
D1, enabling efficient triplet energy transfer from D1 to both types of the acceptors.
Diffusion of the acceptor triplet excitons and the consequent TTA result in generation
singlet excitons and UC fluorescence. For the sample using AC polycrystalline film
as the acceptor, microscopic images of fluorescence from FD1 attached to the HDP
and of UC emission from AC in the vicinity of HDP are shown in Fig. 30.7b, c,
respectively. Clearly, the size of the light spot in the UC emission is larger than that
in fluorescence. The larger size of the UC emission spot results from diffusion of the
triplet excitons within the AC films and the subsequent TTA. In the analysis, the CCD
images were fitted with two-dimensional convolution function of PSF and singlet
exciton density to obtain directly the L T parameters. Examples of one-dimensional
cross sections of the images together with the fits are shown in Fig. 30.7f. The L T
values obtained in this way by measuring UC emission of 46 individual HDPs are
summarized in the histogram in Fig. 30.7h. From the histogram, the mean triplet
exciton diffusion length L T in AC polycrystalline film was 491 nm.
For comparison, we used DPA polycrystalline film as the acceptor. Figure 30.7d,
e shows the microscopic images of fluorescence from FD1 and of UC emission
from DPA, respectively. Figure 30.7g shows horizontal cross sections of the images
together with the fits. The profile of UC emission from DPA is just slightly larger than
that of fluorescence. By analyzing data from 37 individual HDP spot, we obtained a
histogram of L T as shown in Fig. 30.7j. The mean triplet exciton diffusion length L T
in DPA was 172 nm.
The different values of the mean triplet diffusion lengths L T obtained for AC
and DPA, i.e., 491 nm and 172 nm, respectively, can be ascribed to differences
in microscopic and mesoscopic structure of the films. Microscopically, there is a
difference in the π orbital overlap in the triplet state between AC and DPA due to
the different crystal structure. As a result, the Dexter triplet–triplet energy transfer
is more efficient in the AC films. In addition, the AC film is composed of flat crystal
flakes with a size of a few micrometers while the crystal grain size in DPA films
was on the order of tens–hundreds of nanometers. Size analysis of the DPA crystals
measured by SEM is shown as an overlaid histogram in Fig. 30.7j. There is a good
correspondence between the two histograms, and the peak of the size histogram at
213 nm is comparable to the observed L T value of 172 nm. The triplet excitons in
DPA are likely quenched after reaching the grain boundary, and the crystal grain size
is the limiting factor in the triplet exciton diffusion process.
In summary, direct microscopic imaging of triplet exciton diffusion in polycrystalline upconversion materials was demonstrated. A key feature of the method is that
excitation of the donor molecules and generation of high local density of triplet states
are confined into spatial location with sub-diffraction limit size. The triplet exciton
diffusion in the single-component acceptor medium can be studied individually, and
nanoscale location-dependent characteristics of the exciton diffusion process can be
obtained.
533
30 nm) to form the HDP. Triplet energy levels of AC and DPA are lower than that of
D1, enabling efficient triplet energy transfer from D1 to both types of the acceptors.
Diffusion of the acceptor triplet excitons and the consequent TTA result in generation
singlet excitons and UC fluorescence. For the sample using AC polycrystalline film
as the acceptor, microscopic images of fluorescence from FD1 attached to the HDP
and of UC emission from AC in the vicinity of HDP are shown in Fig. 30.7b, c,
respectively. Clearly, the size of the light spot in the UC emission is larger than that
in fluorescence. The larger size of the UC emission spot results from diffusion of the
triplet excitons within the AC films and the subsequent TTA. In the analysis, the CCD
images were fitted with two-dimensional convolution function of PSF and singlet
exciton density to obtain directly the L T parameters. Examples of one-dimensional
cross sections of the images together with the fits are shown in Fig. 30.7f. The L T
values obtained in this way by measuring UC emission of 46 individual HDPs are
summarized in the histogram in Fig. 30.7h. From the histogram, the mean triplet
exciton diffusion length L T in AC polycrystalline film was 491 nm.
For comparison, we used DPA polycrystalline film as the acceptor. Figure 30.7d,
e shows the microscopic images of fluorescence from FD1 and of UC emission
from DPA, respectively. Figure 30.7g shows horizontal cross sections of the images
together with the fits. The profile of UC emission from DPA is just slightly larger than
that of fluorescence. By analyzing data from 37 individual HDP spot, we obtained a
histogram of L T as shown in Fig. 30.7j. The mean triplet exciton diffusion length L T
in DPA was 172 nm.
The different values of the mean triplet diffusion lengths L T obtained for AC
and DPA, i.e., 491 nm and 172 nm, respectively, can be ascribed to differences
in microscopic and mesoscopic structure of the films. Microscopically, there is a
difference in the π orbital overlap in the triplet state between AC and DPA due to
the different crystal structure. As a result, the Dexter triplet–triplet energy transfer
is more efficient in the AC films. In addition, the AC film is composed of flat crystal
flakes with a size of a few micrometers while the crystal grain size in DPA films
was on the order of tens–hundreds of nanometers. Size analysis of the DPA crystals
measured by SEM is shown as an overlaid histogram in Fig. 30.7j. There is a good
correspondence between the two histograms, and the peak of the size histogram at
213 nm is comparable to the observed L T value of 172 nm. The triplet excitons in
DPA are likely quenched after reaching the grain boundary, and the crystal grain size
is the limiting factor in the triplet exciton diffusion process.
In summary, direct microscopic imaging of triplet exciton diffusion in polycrystalline upconversion materials was demonstrated. A key feature of the method is that
excitation of the donor molecules and generation of high local density of triplet states
are confined into spatial location with sub-diffraction limit size. The triplet exciton
diffusion in the single-component acceptor medium can be studied individually, and
nanoscale location-dependent characteristics of the exciton diffusion process can be
obtained.