Optical Properties of Polymer Nanocrystals
The most striking characteristics of the reprecipitation method, as previously described in Chap. 2, is that prepared organic nanocrystals are dispersed in a stable fashion, owing to their highly negative ζ-potential. This fact is technologically importan
- PDF / 2,329,693 Bytes
- 16 Pages / 439 x 666 pts Page_size
- 121 Downloads / 255 Views
14.1
Introduction
The most striking characteristics of the reprecipitation method, as previously described in Chap. 2, is that prepared organic nanocrystals are dispersed in a stable fashion, owing to their highly negative (-potential. This fact is technologically important for some applications. Linear optical properties such as visible (VIS) absorption spectra for poly(DCHD) and perylene nanocrystals and the fluorescence emission spectra from perylene nanocrystals were evidently dependent on their crystal sizes. We will discuss the reason for these interesting optical phenomena in detail, in comparison with the so-called "quantum confinement effect" observed in semiconductor nanoparticles. In addition, we have attempted to fabricate hybridized nanocrystals, which were composed of metal nanoparticles and organic nanocrystals. Namely, poly(DCHD) nanocrystals were individually covered with a gold and/or with a silver thin layer. According to the theoretical prediction [1], the NLO property is expected to be enhanced in these hybridized systems. Finally, we will further demonstrate the enhancement of NLO properties as one of the detailed examples, which is a multilayered poly(DCHD) nanocrystal thin films prepared by utilizing negative (-potential.
14.2 14.2.1
Crystal Size Dependence of Linear Optical Properties Poly(1,6-di(N-carbazolyl)-2,4-hexadiyne) Nanocrystals
Figure 14.1a shows VIS absorption spectra of poly(DCHD) nanocrystals dispersed in an aqueous liquid. The excitonic absorption peak positions, Amax , from the 1f-conjugated backbone of poly(DCHD) chains were clearly shifted to the short-wavelength region with decreasing crystal size [2-4]. The whole relationship between Amax and crystal size is plotted in Fig. 14.1b. On the other hand, the value of Amax = 670nm in fibrous poly(DCHD) nanocrystals was almost the same as that of the corresponding bulk poly(DCHD) crystals [5,6], which suggests that 1f-conjugated polymer chains are aligned along the long axis of the fibrous nanocrystals. H. Masuhara et al. (eds.), Single Organic Nanoparticles © Springer-Verlag Berlin Heidelberg 2003
170
H. Oikawa and H. Nakanishi
3.00
Amax =652
f\,
nm~
f\\
646nm_
"ucos
-€0 2.00
:d=70nm -.- : d=IOO nm ...... : d=150 nm
I \ \
ii i\
«'"
~
\\
\\1\ ...
1.00
"'~""""".".,o 40LO--5-0....0--6--JO'-0--7.:t0""0....,=-'SOO
655 650 645 640 635 20
•
•
•
•
•
•
•
• 40
60
SO 100 120 140 160
Average size / nm
A./nm (a)
(b)
Fig. 14.1. Linear optical properties of poly(DCHD) nanocrystals with three different crystal sizes dispersed in an aqueous liquid: (a), VIS absorption spectra; (b), dependence of excitonic absorption peak positions Amax on crystal sizes
14.2.2
Perylene Nanocrystals
Similar size effects on linear optical properties were also confirmed in perylene nanocrystals [7-9J. The crystal size of perylene nanocrystals was controlled mainly by changing the water temperature in the reprecipitation procedures, and then became smaller with decreasing temperature. Figure 14.2 indicates a typical SEM pho
Data Loading...
