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The as-prepared new kind of homogeneous hybrid organic/inorganic semiconductor nanorods are potential candidates for nanoscale electronic and photonic...
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J. Phys. Chem. C 2008, 112, 8223–8228

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Fabrication of Homogeneous Hybrid Nanorod of Organic/Inorganic Semiconductor Materials Yanbing Guo,†,‡ Yuliang Li,*,† Jinjie Xu,† Xiaofeng Liu,†,‡ Jialiang Xu,†,‡ Jing Lv,†,‡ Changshui Huang,†,‡ Mei Zhu,†,‡ Shuang Cui,†,‡ Lei Jiang,† Huibiao Liu,*,† and Shu Wang† CAS Key Laboratory of Organic Solid, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P.R. China, and Graduate School of Chinese Academy of Sciences, Beijing 100080, P.R. China ReceiVed: December 11, 2007; ReVised Manuscript ReceiVed: March 5, 2008

Homogeneous hybrid organic/inorganic nanorods composed of OPV3 and CdS have been fabricated by a facile template method. The structures of CdS-OPV3 hybrid nanorods have been investigated by field emission scanning electron microscope (SEM), transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and confocal laser scanning microscopy (CLSM). It was found that inorganic semiconductor CdS and organic semiconductor OPV3 homogeneously distributed into the hybrid nanorod. Fluorescence spectroscopy was also used to characterize the optical property of the CdS-OPV3 nanorods. And such prepared CdS-OPV3 homogeneous hybrid nanorods exhibit different optical properties compared with individual component CdS nanorods and OPV3 nanorods. The as-prepared new kind of homogeneous hybrid organic/inorganic semiconductor nanorods are potential candidates for nanoscale electronic and photonic devices. Introduction It is significant in materials science to create nanostructures that combine the optical properties, electrical properties and mechanical flexibility of organic semiconductors with the high stability of inorganic semiconductors.1 As interesting nanostructures, hybrid nanocomposites2 are able to control many physical properties and tune size and shape of nanostructures. Many new concepts for controlling synthesis hybrid nanocomposites have been studied, such as metal-metal junctions,3 metal-polymer junctions,4 inorganic semiconductor junctions,5 inorganic semiconductor-metal junctions6 and semiconductorcarbon nanotube junctions.7 However, how to combine organic semiconductors and inorganic semiconductors for producing the uniformity of the size and shape on nanoscale with unique property is still a challenge. And there has been no report on one-dimensional homogeneous hybrid nanostructures composed of organic/inorganic semiconductor materials. Oligo(phenylenevinylenes) (OPVs) as organic semiconductors have become widely investigated over the past few years due to their applicability to light emitting diodes, field effect transistors and other devices.8 Meanwhile inorganic semiconductor CdS have also been proved to be excellent photoactive materials and charge transmission materials in optoelectronic devices.9 Interestingly, self-assembly hybrid structures of OPVs and CdSe nanocrystals as film devices have been fabricated.10 In this report we would like to describe the concept for controlling the growth of homogeneous hybrid nanorods composed of organic semiconductors and inorganic semiconductors by template method.11 The growth depends on selecting use of the organic unit oligo(p-phenylenevinylene) (OPV3) (Figure 1a) in which two ester groups could benefit by being linked on the surface of inorganic semiconductor CdS for forming of a typical * Corresponding authors. E-mail: [email protected]; [email protected]. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences.

organic/inorganic hybrid interface with ordered arrays and producing different optical properties compared with the single component nanorods. The combination of organic/inorganic semiconductors contains a large amount of homogeneous hybrid interfaces between CdS and OPV3 molecules. The organic-inorganic hybrid interface in the system is able to make the material suitable for applications on photovoltaic devices and light emitting devices.12 Importantly, in our experiment, blue-green and red fluorescent emissions are detected on a single hybrid nanorod that will enable us to exploit them for designing and building novel electronic and photonic devices and other applications. Experiment Method Materials. Sublimed sulfur, dimethyl sulfoxide (DMSO), cadmium acetate, tetrachloroethylene and acetone were purchased from Beijing Chemical Reagent Corporation, China. Octylamine was purchased from Aldrich Corp. The AAO templates with porous diameter of 200 nm and a thickness of 60 µm were purchased from Whatman Co. All of the reagents were used as received. Synthesis of CdS Nanocrystals. CdS nanocrystals were synthesized according to a modified literature method.13 Briefly, 0.25 g of sublimed sulfur (0.007812 mol) was dissolved in 200 mL of dimethyl sulfoxide (DMSO) at 100 °C for about 1 h. After that this solution was heated to 150 °C. Then, a preheated solution of 2.5 g of cadmium acetate (9.398mmol) in 200 mL of DMSO was added. The solution became lemon yellow after several minutes. The reaction solution was heated for about 1 h and then cooled to ambient temperature. CdS nanocrystals were deposited at the bottom of the reaction flask by adding acetone to the solution. Finally, CdS nanocrystal powders were obtained after centrifuging, washing and drying under vacuum at room temperature for about 2 h. The synthesis of organic semiconductor oligo(p-phenylenevinylene) (OPV3) was reported in our previous article.14

10.1021/jp800456c CCC: $40.75  2008 American Chemical Society Published on Web 05/10/2008

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Figure 1. (a) Molecular structure of oligo(p-phenylenevinylene) OPV3. (b) Transmission electron microscope (TEM) image of CdS nanocrystals. (c) Schematic illustration of the template synthesis of CdS-OPV3 hybrid nanorods.

Fabrication of CdS-OPV3 Hybrid Nanorods. The CdSOPV3 hybrid nanorods were fabricated by a simple template method under the assistance of vacuum. Before the synthesis of CdS-OPV3 nanorods, 1 wt% CdS colloid was first prepared by adding 0.1 g of CdS nanocrystals (Figure 1b) and 1 mL of octylamine to 9 mL of tetrachloroethylene. Then, 11 mg of OPV3 (Figure 1a) was added to 2 mL of 1 wt% CdS colloid. In order to acquire a homogeneous CdS-OPV3 solution, the mixture was kept in a flask at 60 °C for about 10 min. The whole synthetic process of CdS-OPV3 nanorods is briefly illustrated in a scheme (Figure 1c). A porous AAO membrane was immersed into the transparent CdS-OPV3 mixture in a flask under reduced pressure. The mixture was exposed to the atmosphere after being kept under reduced pressure for about 5-8 min. And the solution was poured into the pores of the membrane instantly owing to the pressure differentiation.15 Take out and dry the membrane immediately. After repeating the process above three times, CdS-OPV3 nanorods embedded into the channels of the AAO membrane were acquired. Finally, the AAO template was selectively etched by NaOH solution (3 M) and cleaned under the assistance of ultrasonic cleaning machine for later analysis. In order to investigate the different optical properties between hybrid nanorods and the individual component nanorods, individual component CdS nanorods and OPV3 nanorods were also fabricated via the above process. Characterization. The morphologies of the CdS-OPV3 hybrid nanorods and energy-dispersive X-ray spectroscopy (EDS) were observed on a JEOL JSM 6700F field-emission scanning electron microscope, at an accelerating voltage of 15

kV. Transmission electron microscopy (TEM) measurements of CdS nanocrystals were conducted with a JEOL 1011 transmission electron microscope operating at an accelerating voltage of 100 keV. The TEM, HRTEM measurements and selective area electron diffraction pattern (SAED) of the CdS-OPV3 hybrid nanorods were taken with a JEOL 2010 transmission electron microscope using an accelerating voltage of 200 keV. CLSM images were acquired with a WITec CMR200 in the confocal Raman spectra mode. A picosecond pulse laser (405 nm, 135 W, 20 MHz, PIL040F optical head, Advanced Laser Diode Systems, Germany) was used for excitation. The XRD pattern was recorded with a Japan Rigaku D/max-2500 rotation anode X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (k ) 1.5418 Å), employing a scanning rate of 8° min-1 in the 2θ range from 20° to 60°. The UV-vis spectra were recorded on a Hitachi U-3010 spectrophotometer, and the fluorescence spectra were recorded in Hitachi Model F-4500 FL spectrophotometer with an excitation wavelength at 405 nm. Result and Discussion The characterizations of the CdS nanocrystals which have been used to fabricate the hybrid CdS-OPV3 nanorods are shown in Figure 2. From the TEM image (inset in Figure 2), the average diameter of the CdS nancrystals appears to be about 5 nm. The absorption onset of 490 nm in the UV-vis absorption spectrum (blue line in Figure 2) confirms that the average size of CdS nanocrystals is 5 nm.16 The fluorescence spectrum (red line in Figure 2) of CdS colloid shows two fluorescence

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Figure 2. (A) Transmission electron microscope (TEM) image of CdS nanocrystals. (B) Normalized absorption spectrum (in blue) of CdS nanocrystals and fluorescence spectrum (in red) of CdS nanocrystals excited at 405 nm (dispersed in tetrachloroethylene).

emissions: one emission peak is centered at 470 nm; the other ranges from 530 to 790 nm. The fluorescence emission peak centered at 470 nm is the band gap emission of CdS nanocrystals with size ranging from 5 to 14 nm. Compared with the bandedge emission of bulk CdS (ca. 510 nm), the emission peak is about 40 nm blue-shifted because of the quantum confinement effect of CdS nanocrystals. The other strong broad fluorescence emission ranging from 530 to 790 nm is attributed to the surfacedefect emission which is caused by the surface state of CdS nanocrystals such as sulfur vacancies and/or sulfur dangling bonds.17 The morphology of the combined CdS-OPV3 nanorods was observed by scanning electron microscopy (SEM). Figure 3A depicts that the alumina template filled with the CdS-OPV3 nanorods presents a uniform and beautiful bright yellow. Figure 3B shows a large area of the CdS-OPV3 nanorods in a lower magnification. The large amount of nanorods indicated the high filling density of the membrane. The nanorods shown in Figure 3C exhibit the same orientation and monodispersed diameters of 200-150 nm and length of 2-5 µm. The results are in good agreement with the pore diameter of the template. Figure 3D shows the top of two typical CdS-OPV3 nanorods. The successful hybrids of CdS and OPV3 can be demonstrated by the element signature of C, S and Cd in the energy-dispersive X-microanalysis (Figure 3E). Quantitative analysis indicates that the atomic ratio of Cd and S is about 1:1. The Si peak of the spectrum results from the Si substrate. The signature of Pt is the contribution of Pt layer covered for the SEM analysis. The C and O atom signals are detected in the analysis, indicating that the nanorods contain the component of OPV3. On the basis of quantitative EDS analysis of hybrid nanorods (Figure 3E), the ratio of CdS and OPV3 in the hybrid nanorods is about 13:1. To investigate the distribution of CdS nanocrystals and OPV3 in the hybrid nanorods, EDX element maps were collected by scanning the whole CdS-OPV3 hybrid nanorod. The real scanning area on the single CdS-OPV3 hybrid nanorod was labeled by the yellow square in Figure 3F. The respective Cd, S, C and O element maps directly proved the homogeneous hybrid of CdS nanocrystals and OPV3 molecules in the CdS-OPV3 hybrid nanorod. EDX element maps of other nanorods investigated also exhibit similar results. The highly homogeneous hybrid CdS-OPV3 nanorods would be an efficient candidate in the area of energy conversion.

Figure 3. (A) Photograph of the alumina template filled with CdS-OPV3 nanorods. Scanning electron microscope (SEM) images of hybrid CdS-OPV3 nanorods: (B) large area of CdS-OPV3 nanorods after partial removal of the template; (C) well-ordered CdS-OPV3 nanorod arrays under a higher magnification; (D) typical image of CdS-OPV3 nanorods. (E) Energy-dispersive X-microanalysis spectrum (EDS) of the CdS-OPV3 hybrid nanorods. (F) Element mapping of a single CdS-OPV3 hybrid nanorod.

Further structure information and characterization of the CdS-OPV3 nanorods were performed by transmission electron microscopy (TEM) in Figure 4. Figure 4A depicts a few hybrid nanorods of the CdS-OPV3 with diameter of 250 nm and length of 2 µm, which agrees with the SEM images well. The inset in (A) is the selective area electron diffraction pattern (SAED) taken from different areas of the nanorods. Figure 4B displays the end of an independent hybrid nanorod. A HRTEM image (Figure 4C) reveals the exact structure of the nanorod: the singlecrystalline nanoparticles of CdS embedded into OPV3 homogeneously. The two-dimensional lattice part (indicated by the white circle) is CdS nanocrystal. And the amorphous part (indicated by the black circle) is OPV3. The HRTEM image as shown (Figure 4D) supplies precise information on the interface between CdS single-crystal nanoparticle and OPV3. The lattice spacing around 0.204 nm observed in this image agrees well with interplanar distance of the (110) direction parallel in the wurtzite phase of CdS. And the wurtzite structure of CdS nanocrystals can also be proved by XRD (Figure 5). The transition layer of CdS single-crystal nanoparticle and amorphous OPV3, which is about 1-2 nm, also has been clearly exhibited (the area between two black curves in Figure 4D). The XRD patterns for CdS-OPV3 hybrid nanorods and CdS nanocrystals are shown in Figure 5. The hybrid nanorods and CdS nanocrystals have the same diffraction patterns. And all of those peaks can be assigned to (100), (002), (101), (110), (103) and (112) planes of wurtzite structure CdS.18 This information corresponding to the conclusion of HRTEM indi-

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Figure 4. Transmission electron microscope (TEM) images of the CdS-OPV3 hybrid nanorods: (A) TEM image of a few CdS-OPV3 nanorods. The inset in (A) is the selective area electron diffraction pattern (SAED) taken from the nanorods; (B) TEM image of a typical CdS-OPV3 hybrid nanorod; (C) HRTEM image of the CdS-OPV3 hybrid nanorod; (D) HRTEM image of CdS-OPV3 interface under a higher magnification.

Figure 5. X-ray diffraction patterns (XRD) of the CdS-OPV3 hybrid nanorods and CdS nanocrystals.

cates that the hybrid nanorod is composed of wurtzite structure CdS nanocrystals. It is known that nanostructures fabricated by the AAO template wetting method are usually nanotubes.19 In a typical wetting process, a thin film of solution (or polymer melts) wetted and covered the pore walls of the AAO membrane in the initial stage since the cohesive driving force for complete filling is much weaker than the adhesive force. And nanotube formed when evaporation of solvent or cooling led to crystallization or vitrification.20 However, in our experiments, the formation of CdS-OPV3 nanorods experienced a different process. When the AAO template was immersed into the transparent CdS-OPV3 colloid, the poor imbibitions of tetrachloroethylene prevented the thin film formation on the walls of AAO template pores. The CdS-OPV3 colloid was immediately driven into AAO pores by the pressure differentiation. Meanwhile, during the above process, the homogeneous assembly colloid of CdS-OPV3 as a whole had been poured into the AAO template pores. Thus the CdS-OPV3 nanorods prepared by this method show a high homogeneity across the whole nanorod. Based on the fluorescent properties of the two components of CdS and OPV3, confocal laser scanning microscopy (CLSM) was used to confirm the structure of the hybrid nanorods. Figure 6(A, B, C) shows the fluorescent images of a single CdS-OPV3 nanorod (λex ) 405 nm). Figure 6A shows the fluorescent image

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Figure 6. CLSM images (λex ) 405 nm) of a single CdS-OPV3 hybrid nanorod collected (A) from 366 to 929 nm; (B) from 470 to 490 nm; (C) from 557 nm to 651nm. (D) The CLSM fluorescence spectrum of OPV3 nanorods, CdS nanorods and CdS-OPV3 hybrid nanorods.

of the single hybrid nanorod by collecting fluorescent emissions from 366 to 929 nm. Within the resolution of the optical microscope, the nanorod represents a rather uniform intensity of fluorescence. Figure 6B displays that the fluorescent image formed by accepting emissions from 470 to 490 nm presents even the same morphology as Figure 6A. The fluorescence emissions of CdS (bandgap emission) and OPV3 overlap in this area, but the enhanced fluorescence intensity in both the fluorescent image and the fluorescence spectrum demonstrate the contribution of OPV3. So this image confirms that OPV3 distributed into the CdS-OPV3 nanorod uniformly. Figure 6C shows that the fluorescent image formed by accepting the surface-trap emissions of CdS (557-651 nm). This fluorescent image directly depicts that the surface-trap emission of CdS at 557-651 nm is much weaker than the emissions ranging from 470 to 490 nm.21 In fact, the uniform dispersion of CdS nanocrystals in the hybrid nanorod is also confirmed. The successful fabrication of CdS-OPV3 homogeneous hybrid nanorod was rigorously confirmed by the CLSM investigation. We investigated the different optical properties of individual component OPV3 nanorods, CdS nanorods and CdS-OPV3 hybrid nanorods. Figure 6D shows the CLSM fluorescence spectra of CdS nanorods, OPV3 nanorods and hybrid nanorods. As shown in Figure 6D, one of the emission peaks of the hybrid nanorods (red curve) centers at 490 nm, which is different from individual component nanorods of CdS (479 nm) and OPV3 (503 and 528 nm); another is a broad emission peak which ranges from 550 to 700 nm. The emission of hybrid nanorods is stronger than that of CdS nanorods (black curve), and the fluorescence emission of OPV3 is decreased by CdS nanocrystals through the large amount of homogeneous hybrid interfaces in the hybrid nanorods. The different optical properties of single component CdS nanorods, OPV3 nanorods and the hybrid nanorods are also investigated by fluorescence equipment in Figure 7. Figure 7A shows the normalized fluorescence spectra of OPV3 nanorods, CdS nanorods and CdS-OPV3 hybrid nanorods. Due to the large amount homogeneous hybrid interfaces of CdS and OPV3, the characteristic emission of CdS-OPV3 hybrid nanorods (red curve) centers at 499 nm, which is between the peaks of single OPV3 nanorods (blue

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J. Phys. Chem. C, Vol. 112, No. 22, 2008 8227 component nanorods, which may be caused by a large amount of homogeneous hybrid interfaces. The approach is a generalized process that can be used to synthesize different hybrid nanorods of organic semiconductors/inorganic semiconductors with high uniformity of the size and shape, which should have outstanding potential in providing development hybrid structures of ordered nanoscale architectures for photonic electronic and other devices. Acknowledgment. This work was supported by the National Nature Science Foundation of China (20531060 and 20571078) and the National Basic Research 973 Program of China (Grant No. 2006CB932100 and 2005CB623602). References and Notes

Figure 7. (A) The normalized fluorescence spectra of OPV3 nanorods, CdS nanorods and CdS-OPV3 hybrid nanorods. (B) The fluorescence spectra of (A).

curve) and single CdS nanorods (black curve). Under the excitation wavelength of 405 nm, the emission peak of OPV3 nanorods (blue curve) appears at 510 nm, the bandgap emission of CdS nanorods is at the position of 492 nm (black curve) and the surface-trap emission of CdS nanorods ranges from 550 to 700 nm. However the surface-trap emission of CdS nanocrystals in the hybrid nanorods is too weak to be seen in contrast to the band gap emission in the fluorescence spectra. Figure 7B shows the fluorescence spectra of OPV3 nanorods, CdS nanorods and CdS-OPV3 hybrid nanorods. The inset of Figure 7B is the fluorescence spectrum of CdS nanorods (black curve), which is too weak to be seen in one diagram with the fluorescence spectrum of OPV3 nanorods (blue curve). The fluorescence spectra display that the emission of OPV3 decreased greatly in the hybrid nanorods. It is believed that the emission of OPV3 was quenched by CdS nanocrystals. And the fluorescence quenching is indicative of possessing charge transfer between the two semiconductors that is a characteristic process in such a type of inorganic/organic photovoltaic materials.22 The CLSM fluorescence spectra in Figure 6D and fluorescence spectra in Figure 7 demonstrate the obviously different optical property of the hybrid nanorods by comparing with the two individual component nanorods. The results indicate that there are strong interactions between the two components. Conclusion In summary, this work represents a typical fabrication of homogeneous composite nanorods based on the hybrid of organic semiconductor and inorganic semiconductor. The combination of organic/inorganic semiconductor nanorods exhibited a different optical property than the two individual

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