Fibers and Polymers 2008, Vol.9, No.5, 534-537

Preparation of Luminescing Nanocrystal and Its Application to Electrospinning Byung-Soon Kim, Hwan-Moon Song1, Chang-Soo Lee1, Seung-Goo Lee1, and Young-A Son* BK21 FTIT, Department of Organic Materials and Textile System Engineering, Chungnam National University, Daejeon 305-764, Korea 1 Department of Chemical Engineering, Chungnam National University, Daejeon 305-764, Korea (Received March 27, 2008; Revised May 31, 2008; Accepted June 2, 2008) Abstract: In this study, quantum dots (QDs) having the photophysical properties of brightness, photostability and narrow emission were synthesized. The electrospinning has been introduced to be a simple technique for generating ultrathin fibers. Herein, we have synthesized QDs and electrospun polyvinylacetate (PVAc) nanofibers having these strongly luminescing QDs particles. The size and morphology of QDs were recorded with transmission electron microscopy (TEM). The structural nanofiber webs have been discussed by scanning electron microscopy (SEM). And fluorescence properties of strongly luminescing QDs nanofibers were also discussed. Keywords: Quantum dot, Nanocrystal, Strongly luminescing material, Electrospinning, Nanofiber

Introduction

have been synthesized and embedded into PVAc polymer matrix. The fluorescent hybrid fibers with smooth and uniform morphology were then electrospun using electrospinning technique. It is interesting approaches to observe the fluorescence properties of QDs nanofibers. The properties of PVAc/CdSe hybrid nanofibers showing fluorescence effect were discussed.

Quantum dots (QDs) or colloidal nanoparticles have received great attentions in recent years [1-4]. The photophysical properties of QDs, particularly in terms of brightness, photostability, emission color purity and broad adsorption range, are very attractive functions in many applications [5-8]. To our knowledge background, QDs have been enjoyed their applications in bio-probe research fields. This research interest can be raised by the advantages of QDs such as high photoluminescence quantum yields, sharp emission band, long-term photostability and broad excitation spectra. In recent, the uses of nanoparticles embedded in a polymer matrix have been explored and their properties such as photo-activation and strong photoluminescence have been proposed [7]. The electrospinning of nanofibers has been presented to be a simple method for generating ultrathin fiber materials [9-11]. Nanofibers have been considered as very attractive materials due to their large surface areas to volume ratio [9]. This technique is similar to the well known commercial process for drawing microscale fibers except for the use of electrostatic repulsions. In the process of electrospinning, the polymer solution for electrospinning is injected from a small nozzle under the influence of an electric filed and the electrospun fibers are collected on the grounded collector screens [12]. The critical factors for the morphologies and properties of the electrospun nanofibers are polymer concentration, viscosity, applied electric field strength and tip-to-collector distance (TCD). In addition to these variables factors, the humidity and temperature of the surroundings are also considered as important factors. In this study, CdSe nanocrystals as a fluorescence material

Experimental Synthesis of Luminescing CdSe Nanocrystals CdO 0.0127 g and 0.16 g of dodecanoic acid were heated at 200 oC under nitrogen gas. After CdO was completely dissolved the mixture of trioctylphosphine oxide (1.94 g) and hexadecylamine (1.94 g) was added to the flask and heated to 280 oC. At this temperature, a solution containing 0.080 g of Se dissolved in 2 ml of trioctylphosphine was slowly injected into the reaction flask containing CdO mixture. After adding, the temperature was set to 280 oC for the growth of the nanocrystals. After cooling down to 200 oC, the mixture of hexamethyl-sisllathiane (250 μl), diethylzinc (1 ml) and trioctylphosphine (2 ml) was slowly added into the reaction flask. After 1h, the reaction mixture was washed with methanol/chloroform (1:1) to obtain nanocrystals and the obtained CdSe nanoparticles were diluted with chloroform for the storage [1-3]. Electrospinning Process Figure 1 shows a schematic illustration of the basic set up for electrospinning. It consists of three major parts such as a high voltage power supply (a), a metallic needle (b) and a collector (c). The solution for electrospinning was prepared by stirring PVAc and QDs solutions (in the weight ratio 10:4) for 2 h at room temperature. The mixture was loaded into a plastic syringe. A positive terminal was connected to

*Corresponding author: [email protected] 534

Luminescing Nanocrystal and Electrospinning Application

Figure 1. Schematic illustration of the electrospinning process.

the syringe needle tip while aluminum foil covered collector worked as counter electrode. The tip-to-collector distance, the used voltage and winding drum were 20 cm, 20 kV and 10 rpm, respectively. The electrospun PVAc/CdSe hybrid nanofibers were collected on the aluminum foil covering the winding drum.

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of CdSe nanocrystals. The resulting TEM images represent that CdSe QDs were dispersed well and its shapes appeared as approximately spherical. The size of TEM images was observed less than 10 nm in diameter. This size of CdSe nanocrystals is quite satisfactory to embed into electrospun fibers around 1000 nm in diameter. The absorption and emission spectra of CdSe nanocrystals were recorded with Agilent 8453 spectrophotometer and Shimadzu RF-5301 spectrofluorophotometer, respectively. The corresponding absorption and photoluminescence spectra of CdSe QDs in chloroform are shown in Figure 3. As mentioned previously, the broad absorption spectra of QDs, one of the important characteristics, could allow for the efficient excitation of different sized QDs-based fluorescence materials with a single light source. Emission spectra peaks of QDs (I) and QDs (II) were observed at 587 nm and 633 nm, respectively. The emission states of QDs solution showed yellow and red fluorescence properties, where the fluorescence emission colors are related to the size increase of QDs. The band gap of CdSe nanocrystals decreases as their size

Results and Discussion The size of CdSe QDs was recorded with transmission electron microscopy (TEM). TEM was determined with JEM-2010 at 200 kV. Figure 2 shows the typical TEM images

Figure 2. TEM images of CdSe QDs.

Figure 3. Absorption and photoluminescence spectra of CdSe QDs.

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increases and thus the emission color of the photoluminescence of these nanocrystals shifts from yellow (centered 587 nm) to red (centered 633 nm) as the size of the nanocrystals increases. Thus, the control of the size is the major key factor on the color purity of the emission [13]. Due to the failure and difficulty in controlling and tuning the QDs size the efforts to produce PVAc/CdSe fluorescent nanofibers were mainly focused on the study using 587 nm (yellow) and 633 nm (red). The electrospun PVAc/CdSe nanofibers were successfully prepared using electrospinning in the acetone/chloroform solution. Surface morphologies and dimensions of PVAc/ CdSe nanofibers were observed using JSM-7000F SEM. The SEM images of electrospun PVAc/CdSe nanofibers with their weight ratios in experiment solution of 10:4 are shown

Byung-Soon Kim et al.

in Figure 4. At this result, the uniform and super-long lengths of composite nanofibers were formed on the aluminum foil. The diameter of the electrospun nanofibers lies approximately between 900 to 1500 nm. In all case, the electrospun nanofibers are randomly oriented and webbed on the aluminum foil without any bead-on-string morphology. The hybrid nanofibers exhibit a smooth and uniform surface morphology, where the QDs nanoparticles are well embedded in PVAc polymer matrix. The addition of CdSe QDs inorganic particles to the spinning polymer solution did not affect any adverse effects to form smooth and regular fiber webs. Figure 5 shows the fluorescence images of electrospun PVAc/CdSe nanofibers. The determination of fluorescence emission was used by 365 nm UV-light tubes. The electrospun nanofibers with yellow and red fluorescence were observed due to the photoluminescent properties of size dependant CdSe QDs. An equal and uniform luminosity were shown throughout whole webbed electrospun nanofibers.

Conclusion

Figure 4. SEM images of electrospun CdSe QDs nanofibers.

In this study, CdSe nanocrystals as a fluorescence material have synthesized and embedded into PVAc polymer matrix. The emission spectra of QDs (I) and QDs (II) were obtained at 587 nm (yellow) and 633 nm (red). The fluorescence and emission color properties are dependant on the size of QDs nanoparticles. Using the electrospinning method, PVAc nanofibers having strongly luminescing CdSe QDs with diameter approximately between 900 nm to 1500 nm were obtained. The nanofibers with yellow and red fluorescence were observed due to the emission property of CdSe nanocrystals. This electrospinning technique offers a simple method for generating PVAc/CdSe hybrid nanofibers.

Acknowledgement This research was financially supporting by the Ministry of Education, Science Technology (MEST) and Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Regional Innovation.

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Figure 5. Fluorescence images of electrospun CdSe QDs nanofibers.

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Luminescing Nanocrystal and Electrospinning Application

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