Page 584 - Book Hosokawa Nanoparticle Technology Handbook

APPLICATIONS 32 DEVELOPMENT OF OPTICAL MEMORY
(such as ZnS, ZnSe, CdS, etc.) is coated around the 3. Methods of preparing and evaluating CdSe
core part of the nanoparticles. thin films
Some other features of nanoparticles are that it is
possible to generate a wide range of optical charac- CdSe/ZnS core/shell-type nanoparticles with a parti-
teristics by changing the material of the shell part, cle diameter of about 4 nm were coated on a glass
and that it is possible to control over a wide range substrate using the spin coating method. The nanopar-
the hydrophobicity, the hydrophilicity, and pH ticle thin films were observed under a nitrogen gas
durability, etc., by changing the surface-active atmosphere using a confocal laser scanning micro-
2
agent. It is also possible to modify the functional scope. First, a 60 m area was observed with an irra-
moiety that reacts to a specific molecule or DNA, diation intensity of 0.6 nW (read-out operation). At
and because of the characteristic that color quench- this time, the observed fluorescence intensity is uni-
ing is less likely to occur compared to the fluores- form (Fig. 32.2a). Next, an excitation light of 11, 220,
2
cent dyes being used at present, these nanoparticles or 1,370 nW was emitted onto a 7.4 m area at the
are expected to be applied in the medical field as center (writing operation). After that, the observation
2
biological labeling agents [6–8]. Further, since the area is returned to 60 m , and this area was observed
specific surface area of nanoparticles having diam- again with an irradiation intensity of 0.6 nW
eters of 10 nm or less is large, the optical charac- (Fig. 32.2b). At this time, it was observed that the flu-
teristics are extremely sensitive to the surrounding orescence intensity of the central part in which the
environment. Application to sensors using this writing operation was made had increased. The read-
characteristic has also been studied [9, 10]. out operation at 0.6 nW and the writing operation
However, how the boundary surface of nano sizes using a strong excitation light were repeated, and the
affects the optical characteristics is not yet fully changes in the value of the fluorescence intensity at
explained. the center during the read-out operation were
observed. In other words, the fluorescence intensity at
the central part in which the writing operation was
2. Optical memory effect of semiconductor carried out was taken as I, and the fluorescence inten-
nanoparticle thin films sity at the surrounding part in which no writing oper-
ation was carried out was taken as I , and the ratio I/I 0
0
The present authors have reported [11–13] a phenom- was obtained as an index of the fluorescence intensity.
enon that, when CdSe nanoparticles are coated on a The wavelength of the excitation light used in all the
substrate thereby preparing a thin film, and excitation operations was 488 nm.
light is irradiated on the thin film continuously, the
intensity of fluorescence light from the thin film 4. Dependency of intensity of fluorescence on the
increases up to a specific value.
If the thin film whose fluorescence intensity has excitation light intensity
increased is stored in a dark place, and after a specific The dependency on the excitation light intensity of
period of time, if it is irradiated again with excitation the behavior of I/I is shown in Fig. 32.3. The
0
light, the fluorescence intensity will be almost the horizontal axis represents the total energy radiated
same value as the fluorescence intensity before stor- during the writing operation, and the vertical axis
ing in the dark place. In other words, this phenome- represents the value of I/I during the read-out oper-
0
non can be said to be that “the excitation light ation. When an excitation light of an intensity of
irradiation time interval is being recorded as the fluo- 11 nW is used during the writing operation, the flu-
rescence intensity”. orescence intensity I increases successively as the
The possibilities of multiple-value recording using
the continuous increase in the fluorescence intensity
or of recording in smaller areas using near-field light (a) (b)
have already been reported [12], and it is expected to
apply these phenomena to optical memories and to
realize higher recording capacity than conventional
methods. In addition, since these samples are pre-
pared by a coating process, there is also another big
advantage that it is possible to prepare large-area
films on flexible substrates.
In this paper, we report that the recording and read-
out operations of information in the form of fluores- 7.4μm 7.4μm
cence intensity were actually carried out, that these
operations could be made with a single-wavelength Figure 32.2
light, and that reversible changes of the recorded (a) Fluorescence image before writing operation.
value are possible [11]. (b) Fluorescence image after writing operation.

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