Optimal Fluorescence Property of CdSe Quantum Dots and Electrospinning Polyvinylpyrrolidone Hybrid Microfibers

ZHANG Xin-bo,CONG Long-zhuang,YANG Lan-lan,DU Zhong-lin,WANG Yao,WANG Yan-xin,HUANG Lin-jun,GAO Fan,Laurence A. Belfiore,TANG Jian-guo*

1. Institute of Hybrid Materials,National Center of International Joint Research for Hybrid Materials Technology,National Base of International Sci. & Tech. Cooperation on Hybrid Materials,Qingdao University,Qingdao 266071,China 2. Department of Chemical and Biological Engineering,Colorado State University,Fort Collins,Colorado 80523,USA

Abstract Semiconductor nanocrystals (NCs) have been widely researched and reported in the past few years due to their excellent light stability,wide emission persistence and high extinction coefficient. Among them,CdSe NCs are widely used in electronic lighting,solar power generation,photoelectric sensing and other fields. However,the electrical,thermodynamic and photophysical properties of CdSe NCs have a size dependence,crystal surface defects and dangling bonds,and serious biological and environmental toxicity are prone to occur in traditional preparation methods applications,which limit their direct application. To realize the application of quantum dots in various fields,the emission wavelength,size distribution and fluorescence properties of CdSe NCs must be strictly controlled. In this study,monodisperse colloidal luminescent CdSe quantum dots were synthesized by high-temperature thermal injection method,and CdSe NCs were modified with surface ligands,and the effects of ligands with different alkyl chain lengths on the size distribution and fluorescence properties of CdSe NCs were studied. In addition,the spinning solution was prepared by changing the solvent ratio and hybridized with polyvinylpyrrolidone (PVP) to prepare PVP/CdSe QDs hybrid fibers. The results show that the CdSe NCs modified by surface ligands have good stability in the organic solution due to the decrease of intermolecular adsorption the modification of surface ligands,as well as adjustable solubility,which compensate for defects and dangling bonds caused by the decline in fluorescence performance,and play an important regulatory role in the formation of CdSe crystal structure. More importantly,this study combines surface ligand modification and hybridization to improve the adhesion of surface ligands and avoid direct contact between cadmium selenide nanocrystals and the polymer matrix during the preparation of hybrid materials. The fluorophore provides a good microenvironment and ensures the fluorescence performance of CdSe NCs,and the hybrid fiber also has stable fluorescence performance. The introduction of PVP has reduced the biotoxicity and environmental toxicity of CdSe NCs,made the material more environmentally friendly has better biocompatibility,and greatly increased the material’s application range. The experimental results show that PVP/CdSe QDs hybrid microfibers have good hybrid compatibility and dispersion,excellent fluorescence performance and material formability,simple synthesis ways and low cost,and applied to solution processing,optical lighting,Electrode materials,and biological imaging and other fields.

Keywords CdSe； Polyvinylpyrrolidone； Electrospinning； Fluorescence

Introduction

Semiconductor nanocrystals (NCs) have attracted great interest over the past years due to their unique properties (i.e.,photostability,broad emission tenability and,high extinction coefficient). Usually,QDs NCs mainly consist of Ⅳ,Ⅱ-Ⅵ,Ⅳ-Ⅵ or Ⅲ-Ⅴ elements,such as Ⅱ-Ⅵ QDs (CdSe,CdTe,ZnSe,ZnTe etc.)[1],Ⅲ-Ⅴ QDs (InP,InAs)[2],and Ⅳ-Ⅵ QDs (PbS,PbSe)[3]. Among them,CdSe NCs were widely used to light-emitting diodes (LEDs),solar cells,nanolasers[4],waveguides,photosensors,and photodetectors[5]. In order to realize the application of quantum dots in various fields,the properties of CdSe NCs (i.e.,emission color,size distribution,fluorescence intensity and stability) must be strictly controlled. Usually the regulation is achieved using surface ligands. Surface ligands have an extremely important influence on these properties of quantum dots,which can be divided into the following three aspects: (1) The influence of surface ligands on fluorescence properties[6]. The surface defects and dangling bonds of nanocrystals cause energy distribution inside and outside the crystal,resulting in a decrease in fluorescence efficiency. The combination with surface ligands can compensate for performance degradation caused by the defects and dangling bonds. (2) The influence of surface ligands on the internal structure of NCs[7]. The ligand-surface interaction plays an important role in the formation of CdSe crystal structure. (3) The influence of surface ligands on the solubility of NCs[8]. Different ligands can cause significant differences in the solubility of NCs. The interaction between long-chain ligands will reduce the solubility of NCs.

CdSe NCs modified with surface ligands have good optical properties. However,various problems often occur in practical applications. For example,the instability of the microenvironment leads to the desorption of surface ligands,which reduces the fluorescence performance,and the leakage of packaging leads to Biological toxicity and environmental pollution. Therefore,under the premise of ensuring its luminous performance,finding a safe,simple and easy application method is a subject of extremely research significance. In recent years,there have been a large number of reports on quantum dot solar cells,but few reports on luminescent materials that are closer to life. Electrospinning is a technology widely used in electrostatic fiber forming,which uses high-voltage power to produce polymer fibers with diameters ranging from 2 nm to several microns[9]. Compared with other spinning technologies,electrospinning technology can produce microfibers with a smaller diameter and a larger surface area and been widely studied in the past few years,such as solution filtration[10],optical illumination[11],electrode materials[12]and biological imaging[13]. At present,the electrostatic spinning process mainly uses the solution spinning method,and the boiling point of the solvent has a great influence on the spinning process[14]. The solvent determines the formation of microfibers and fluorescent properties. Therefore,it is very important to find an appropriate solvent.

In this study,CdSe QDs/PVP hybrid ultrafine fibers with excellent fluorescence properties were successfully prepared by electrospinning technology. Monodisperse CdSe quantum dots are synthesized by a low-cost and simple method. In the synthesis process,surface ligands with different lengths of alkyl chains were used for modification. The results show that the surface ligand module affects the diffusion and fluorescence intensity of molecules. Hybridization of inorganic CdSe quantum dots modified with surface ligands with polymers can surface ligands’ desorption,and make CdSe QDs have good dispersibility in PVP molecules,which solves the problem of material blending formability.

1 Experimental

1.1 Materials

Cadmiumoxide (CdO,99.99%),Hexadecyl-amine (HDA,98%),Octadecene (ODE,90%),Octadecyl-amine (ODA,98%),Trioctylphosphine (TOP,99%),Selenium powder (Se,99.99%),Polyvinylpyrrolidone (PVP,Mw=1 300 000) were purchased from Aladdin. The dichloromethane (CH2Cl2),methanol,anhydrous ethanol were purchased from McLean. All reagents were used as received and no further purified.

1.2 Synthesis of CdSe QDs NCs

The CdSe QDs were prepared in ODE and the details are shown as followed. CdO (0.012 8 g,0.1 mmol) were mixed with 2 mL ligands (HDA or ODA) and 8 mL ODE. The reaction mixture was degassed under vacuum for 20 min at 120 ℃. After that,Se stock solution was prepared as follows. The mixture of Se (0.007 8 g,0.1 mmol),1 mL of TOP and 1 mL of ODE was degassed for 30 min. Next,the Se was dissolved by ultrasonic until a clear colorless solution was formed. The obtained Se stock solution for injection. Then,the solution was heated to 300 ℃ for 20 min under nitrogen protection and until a colloidal solution was formed. Then,the temperature of the solution is reduced to 260 ℃,and 2 mL Se stock solution was added dropwise into the reaction mixture. After the injection,the reaction mixture keeps growth at 240 ℃. The resulting samples were centrifugal purification with methanol for subsequent use.

1.3 Electrospinning based on QDs/PVP hybrid materials

The CdSe QDs/PVP hybrid microfibers were prepared via the electrospinnig technique. And the precursor solution for electrospining was prepared as follows. Firstly,PVP (1 g,10%Wt) were mixed with 8 mL ethanol into a sealed beaker and stirred for 12 h at room temperature. Next,The 1 mL CdSe QDs were added dropwise to the spinning solution and continue to keep stirring for 1 h until mixed completely. The spinning solution is placed into 5 mL of the needle tube. Then,the needle tube is placed in the electrospun machine. To obtain microfibers,spinning was carried out for 1 hour under the condition of 16 kV voltage,room temperature and the air humidity of 40%.

1.4 Characterization

All-optical measurements were carried out promptly (within 12 h after the synthesis). All the CdSe NCs samples were measured with excitation wavelength at 450 nm. CdSe QDs were characterized by transmission electron microscopy (TEM,JEOL2011),UV-Visible spectrometer (Lambda 750),luminescence spectrometer (Hitachi U-4100),high-resolution transmission electron microscopy (HTEM,JEM-2100F). Microfibers were characterized by scanning electron microscopy (SEM,JEOL 6460),laser scanning confocal microscopy (Flou ViewTM FV1000),Electrospinning machine (TEADFS-100).

2 Results and Discussion

2.1 Morphology and Size distribution with different surface ligands

Monodispersed CdSe QDs were synthesized by a simple and safe method. The ligands of different alkyl chain were changed during the synthesis process. The results exhibited that the size dispersion and fluorescence intensity were influenced by the surface ligands modules.

Fig.1 Morphology and Size distribution of CdSe QDs respectively usingHDA (a and c,e) and ODA (b and d,f) at 15 min

Figure 1 presents the size distribution of CdSe NCs with two different ligands (HDA and ODA). It is seen from Figure 1 (a) and (b) that the size of the QDs made by using ODA is unigorm. It is seen from Figure 1 (e) and (f) that the size distribution of QDs synthesized by using HAD is from 4.6 nm to 6.4 nm,which is mainly in the 5.8 nm to 6.1 nm. The average size is about 5.9 nm. The size distribution is not very uniform. The size distribution of QDs synthesized by using OAD is from 4.6 nm to 5.7 nm,which is mainly in the 5.2 to 5.4 nm. The average size is about 5.2 nm. Compared with the size distribution of QDs synthesized by using HAD,the distribution of QDs synthesized by using OAD is more uniform. This is because the long-chain surface ligands have greater steric hindrance and stronger interactions,resulting in uneven size distribution. It demonstrates that short-chain surface ligands have a better effect on crystal size control.

2.2 Optical properties of CdSe QDs with different surface ligands

Figure 2 (a) and (b) present the temporal evolution of the absorption and luminescence spectra of colloidal CdSe NCs grown with two different ligands (HDA and ODA). With the increase of reaction time,the emission peak position and bandgap absorption of the QDs synthesized using the same ligands continuously move to a long wavelength. Compared with the QDs synthesized by using HDA,the emission peak position and bandgap absorption of QDs by using ODA showed a significant blue shift at the same reaction time. Due to the quantum confinement effect of QDs,The larger the emission peak position of the QDs,The larger the QDs size. As shown in Figure 2 (d),the emission peak position of the QDs synthesized by using HAD is always bigger than the emission peak position of QDs synthesized by using ODA at the same reaction time. It shows that the QDs of using short-chain fatty amine as ligands have a larger size than QDs of using the long-chain fatty amine as ligands in the same conditions. The ligands play important roles in controlling the size of QDs.

Fig.2 Emission spectra of CdSe QDs using HDA and ODA (a)； Absorption spectra of CdSe QDs using HDA and ODA (b)； Evolutionary trend of the HWFM (c)； Evolutionary trend of the peak (d)

Usually,the effects of ligands on the formation of CdSe QDs can be divided into two aspects,the effects on the nucleation of monomer and the growth of QDs[15]The precursor quickly converted into active monomer in the nucleation stage when the reaction system at high temperature. It will lead to nucleation when the monomer concentration over saturation. The effect of ligands on the activity of the monomer plays a leading role in this process. The binding capacity of ligands and monomer and steric hindrance of ligands have greater effect on monomer activity. The longer the carbon chain of the ligands,the greater the steric hindrance. Ligands with a large steric hindrance will reduce the diffusion of Cd2+,which lead to activity reduction of Cd2+.

In the growth stage,the concentration of monomer decreases in the solution and the remaining monomer will continue to bind to the nuclear surface,causing the nucleation to grow. At this time,the ligands’ adhesion to the surface of the growing NCS is the most important factor in affecting crystal growth. When the ligands deviated from the NCs surface,the remaining monomers will grow at the crystal surface’s corresponding sites in the growth temperature. The stability of different types of ligands on the quantum dot surface is different. Compared with ODA,the adhesion of the HDA to the surface of the growing NCs is smaller. It means that Cd2+of using ODA as surface ligands will be more difficult to combine with the nuclear surface. The growth of QDs is hampered,which caused the size of QDs to become small. Therefore,compared with the QDs synthesized by using HDA,the QDs synthesized by using ODA as ligands are smaller under the same conditions.

2.3 Effect of Ligands on Fluorescence Properties of QDs

Figure 3 showed that QDs synthesized with ODA have a higher fluorescence intensity than QDs synthesized with HDA at the same concentration of QDs. Due to the smaller size of the QDs,a large number of atoms are on the surface of the crystal. Such a large surface-to-volume ratio causes many crystal defects on the surface of the QDs,leading to a decrease in the fluorescence intensity of the QDs. Surface ligands can reduce defects on the surface of QDs,which leads to an increase in the fluorescence intensity of the QDs. The surface ligands will fall off in the process of purification and dilution of CdSe QDs,which results in a decrease in fluorescence intensity. The stability of different types of ligands on the QDs surface is different. Compared with ODA,the adhesion of the HDA to the surface of the growing NCs is smaller. Because of that HDA has lower adhesion to growing surfaces of QDs NCs than ODA. HDA can be more easily detached from the surface of the QDs NCs in the process of purification and dilution of CdSe QDs. That leads to more defects in the surface of QDs synthesized using HDA. Therefore,the fluorescence intensity of QDs synthesized using HDA is lower.

Fig.3 Emission spectra of CdSe QDs using HDA and ODA in the same reaction time and the same concentration

2.4 Preparation of QDs/PVP Hybrid microfibers

In the process of making a spinning solution,the solvent of pure CH2Cl2is volatilized too fast,leading to blockage of the spinneret. It makes the spun fiber with poor morphology. The solvent of pure alcohol causes QDs to fluoresce quenching quickly. Therefore,CH2Cl2and ethanol are mixed as a solvent. Not only ensure that solvents are not volatilized too fast to spinning,but also ensure that the QDs will not fluoresce quenching.

Fig.4 EDS diagram of PVP microfibers (a),and QDs/PVP microfibers (b)

Table 1 Element content of PVP microfibers (a),and QDs/PVP microfibers (b)

Figure 4(c) and Table 1 show that Cd and Se are not present in pure PVP fibers. Figure 4(d) and Table 1 show that QDs/PVP microfibers contain Se and Cd elements. This indicates that CdSe QD was successfully mixed into the spinning suspension. The EDS spectrum shows that the element ratio of CdSe is not 1:1. This is because the ligand’s polar group forms a coordination bond with CdSe,while the lactam group of the PVP matrix has strong polarity and is easy to interact with the surface. The interaction between the ligands leads to the difference in the cadmium selenide unit’s orientation on the surface of the matrix. This further proves that the cadmium selenide quantum dots in the PVP fiber can still have good surface adhesion with the surface ligands,and will not cause desorption during the doping process to cause changes in the photophysical properties,which shows that the PVP fiber Hybridization effectively improves the defect of surface ligand detachment during HAD purification and dilution.

Fig.5 Fluorescence microscope picture of CdSe QDs using ODA

Figure 5 shows that the microfiber diameter is mainly distributed in the range of 900～2 000 nm,and the thickness distribution is relatively uniform. This shows that the spinning process is relatively well-controlled and the spinning solution has a moderate viscosity. This ensures that there will be no polymer agglomeration and insoluble in the spinning solution,and makes the doping of quantum dots more uniform. The results can be seen from the fluorescence microscope,four quantum dots with different emission wavelengths are uniformly doped into the fiber,and have excellent fluorescence performance.

3 Conclusion

In this study,CdSe quantum dots were synthesized using CdO and Se and two different fatty amines (ODA,HDA) as ligands in the non-coordinating solvent EDO by a simple and easy method. Spectral analysis proves that the ligand carbon chain’s different lengths lead to different steric hindrance,which affects the monomer reactivity and the adhesion of the surface ligand to CdSe. The ligand’s carbon chain length can effectively control the size and size distribution of QDs and the fluorescence intensity. CdSe quantum dots with narrow size distribution and high fluorescence intensity are obtained by the above method. PVP/CdSe quantum dot hybrid microfibers prepared by electrospinning using PVP and high-quality CdSe quantum dots as raw materials have the advantages of variable luminescence wavelength,high luminous intensity,and reusable use. It has important potential applications in optical lighting,electrode materials,biological imaging and other fields.