Ag nanoparticles/PPV composite nanofibers with high and sensitive opto-electronic response
© Chen et al; licensee Springer. 2011
Received: 28 April 2010
Accepted: 7 February 2011
Published: 7 February 2011
The novel Ag nanoparticles/poly(p-phenylene vinylene) [PPV] composite nanofibers were prepared by electrospinning. The transmission electron microscope image shows that the average diameter of composite fibers is about 500 nm and Ag nanoparticles are uniformly dispersed in the PPV matrix with an average diameter of about 25 nm. The Fourier transform infrared spectra suggest that there could be a coordination effect to a certain extent between the Ag atom and the π system of PPV, which is significantly favorable for the dissociation of photoexcitons and the charge transfer at the interface between the Ag nanoparticle and the PPV. The Au top electrode device of the single Ag/PPV composite nanofiber exhibits high and sensitive opto-electronic responses. Under light illumination of 5.76 mW/cm2 and voltage of 20 V, the photocurrent is over three times larger than the dark current under same voltage, which indicates that this kind of composite fiber is an excellent opto-electronic nanomaterial.
Recently, 1D opto-electronic nanomaterials, especially the 1D organic opto-electronic nanomaterials, have received much attention of scientists because of their distinctive geometries, novel opto-electronic properties, and the potential application in nano/micro devices [1–5].
Electrospinning is an efficient technique for the fabrication of 1D polymer-based nanomaterials. Up to now, a lot of polymers and polymer-based composite materials have been fabricated by electrospinning [5–7]. Poly(p-phenylene vinylene) [PPV] is a typical conjugated polymer which has good photoluminescent [PL] and electroluminescent properties as well as photovoltaic and nonlinear optical properties [8–10]. Our research group has successfully fabricated the PPV nanofibers and the PPV-based composite nanofibers by electrospinning, such as TiO2/PPV and CdSe/PPV nanofibers, etc., which showed novel opto-electronic properties [11, 12].
Metal nanomaterials exhibit many novel physical and chemical characteristics which arise from their quantum confinement effects and their enormously large specific surface areas. Therefore, metal nanomaterials are used as a kind of block to build advanced functional materials or to improve the efficiency of devices in many researches. Lee et al.  reported that the incorporation of gold nanodots on the indium tin oxide surface can obviously increase the power conversion efficiency of poly(3-hexylthiophene)/-phenyl C61-butyric acid methyl ester solar cell. Nah et al.  reported that the electrochromic absorption was markedly enhanced in Ag nanoparticles embedded in MEH-PPV composite films. The opto-electronic response of the pristine PPV film device is relative low , which makes the investigation of the opto-electronic character of a single PPV nanofiber difficult. We expect that incorporating Ag nano-particles in PPV nanofibers can prepare a novel composite nanofiber with a high opto-electronic response.
In this paper, Ag nanoparticles/PPV composite nanofibers were successfully prepared by electrospinning. Then, the Au top electrode device of a single composite nanofiber was fabricated on a SiO2 substrate by an 'organic ribbon mask' technique, which showed high and sensitive opto-electronic response.
Preparation of Ag nanoparticles and Ag/PPV composite nanofibers
Sodium borohydride (NaBH4) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), while ethanol and silver sulfate (Ag2SO4) were from Beijing Beihua Fine Chemicals Co., Ltd. (Beijing, China). All reagents were of analytical grade and used without further purification.
The synthesis process of Ag nanoparticles/PPV composite nanofibers is as follows: Firstly Ag2SO4 (0.21 g, 0.67 mmol) was dissolved in 100 ml distilled water to get a clear solution, and then NaBH4 (1.40 g, 0.037 mol) was added into the solution with vigorous stirring under N2-saturated atmosphere. After filtering, drying, and triturating, we obtained Ag nanoparticles. Then, 3 mg of Ag nanoparticles was added into 0.4 wt.% PPV precursor ethanol solution (2.63 g) with stirring at room temperature for 24 h to obtain a new solution. Then, the solution was electrospun at room temperature, with positive voltage of 15 kV, humidity of 45%, and tip-to-collector distance of 20 cm. Finally, the electrospun fibers were heated at 180°C for 4 h in a vacuum oven for conversion of the PPV precursor to PPV.
The pristine PPV nanofibers were also prepared in a similar procedure as described above.
The small-angle X-ray diffraction [SAXRD] measurements were performed on a small-angle X-ray diffractometer (PX13-010, Japan). Fourier transform infrared [FTIR] measurements were carried out on a Fourier transform infrared spectrometer (Magana 560, Nicolet Corp., Madison, WI, USA). The photoluminescence excitation [PLE] and PL measurements were made on an Eclipse Fluorescence Spectrophotometer (Varian Corp., Palo Alto, CA, USA). The morphology of nanofibers was observed using a transmission electron microscope [TEM] (FP 5021/20, Czech Republic). A scanning electron microscope [SEM] (ESEM XL-30, FEI Company, Hillsboro, OR, USA) was used to reveal the structure of the Au top electrode device of a single Ag/PPV composite nanofiber. The opto-electronic response of this device was measured with a Keithley 4200 SCS and a Micromanipulator 6150 probe station in a clean and shielded box.
Results and discussion
Opto-electronic characteristics of the single composite fiber device
It is noticed that the two I-V curves under light illumination of 3.04 and 5.76 mW/cm2 are close to each other before 10 V, e.g., the phenomenon of photoresponse saturation happens, which could be related to the charge accumulation and de-trapping by light effects at the contacts.
The Ag nanoparticles/PPV composite nanofibers with an average diameter of 500 nm were prepared by electrospinning. The TEM image shows that the Ag nanoparticles with an average diameter of 25 nm were dispersed uniformly in the PPV matrix. It was deduced from the FTIR spectra that there was a complexation between the Ag atom and the π system of PPV, which should be significantly favorable for the charge separation of photoexcitons and the charge transport at the interface between the Ag particles and the PPV. The J-V measurement of the device under light illumination with different intensities shows that the Ag nanoparticles/PPV composite nanofibers have high and sensitive opto-electronic response and will have good potential application in the micro/nano organic opto-electronic field.
We thank Prof. Wenping Hu and Dr. Yajie Zhang (Beijing National Laboratory for Molecular Sciences and Key Laboratory of Organic Solids, Institute of Chemistry Chinese Academy of Sciences) for the fabrication of the Au top electrode device and the measurement of I-V curves. The research described herein was supported by the National Natural Science Foundation (grant no. 2077 4017), Natural Science Foundation of Heilongjiang Province of China (grant no. B200606), and Analysis and Testing Foundation of Northeast Normal University.
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