Reprecipitation Method for Organic Nanocrystals
Nanoparticles and/or nanocrystals in semiconductors and metals have been extensively investigated from the viewpoints of both science and industry, e.g., thermodynamics, crystal structures, optical properties, and reactivity as catalysis [1–4]. Nanocrysta
- PDF / 2,264,417 Bytes
- 15 Pages / 439 x 666 pts Page_size
- 81 Downloads / 153 Views
2.1
Introduction
Nanoparticles and/or nanocrystals in semiconductors and metals have been extensively investigated from the viewpoints of both science and industry, e.g., thermodynamics, crystal structures, optical properties, and reactivity as catalysis [1-4]. Nanocrystals are located in a mesoscopic phase between a single molecule and/or atom and the corresponding bulk crystals [3-6]. Interestingly, it was reported that nonlinear optical (NLO) properties were enhanced on the basis of the quantum confinement effect in semiconductor nanoparticles with sizes below lOnm [7-14]. These nanoparticles in inorganics could be fabricated either by deposition methods in a molten glass matrix or by vacuum-evaporation processes [15]. On the other hand, organic compounds have an essentially high functional potential in their physicochemical properties [16], in comparison with inorganic materials. In other words, organic nanocrystals are expected to exhibit various novel optical and electronic properties, and some applications will be realized in the fields of electronics and photonics devices. However, fabrication processes for inorganic nanoparticles could not be applied conveniently to thermally unstable organic compounds. Actually, little attention had been paid so far to organic nanocrystals, when our scientific project on organic nanocrystals started [17,18]. In contrast to fabrication techniques for inorganic nanoparticles, we demonstrated that the "reprecipitation method" was available and convenient to prepare some kinds of n-conjugated organic nanocrystals [19]: polydiacetylene (PDA) derivatives [20-22], low-molecular weight aromatic compounds such as perylene and C 60 [23-26], and organic functional chromophores, e.g., pseudo-isocyanine, merocyanine and phthalocyanine [27,28]. In general, the crystal size of the obtained organic nanocrystals was in the range of several tens of nanometers to submicrometer [20,23,28]. Some interesting experimental results have been confirmed depending on crystal size. The conversion of solid-state polymerization in diacetylene monomer nanocrystals was enhanced at the optimal crystal size [29]. In addition, the excitonic absorption peak positions were shifted to the short-wavelength region with decreasing crystal size in perylene and PDA nanocrystals [20-22], whereas the emission H. Masuhara et al. (eds.), Single Organic Nanoparticles © Springer-Verlag Berlin Heidelberg 2003
18
H. Nakanishi and H. Oikawa
peak from the free-exciton (F-exciton) state in perylene nanocrystals appeared gradually with decreasing crystal size, and its position was also shifted subsequently to the high-energy region [23-25]. In the present chapter, we describe certain kinds of reprecipitation methods for fabricating organic nanocrystals, and we will also discuss nanocrystallization processes to control the crystal size and shape.
2.2 2.2.1
Preparation of Organic Nanocrystals by the Reprecipitation Method Routine Reprecipitation Method
In the routine reprecipitation method [20-27] as shown in Fig. 2.1,
Data Loading...
