Fabrication of Homogeneous Hybrid Nanorod of Organic/Inorganic SemiconductorMaterials

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), Instituteof Chemistry, Chinese Academy of Sciences, Beijing 100080, P.R. China, and Graduate School of ChineseAcademy 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 afacile template method. The structures of CdS-OPV3 hybrid nanorods have been investigated by field emissionscanning electron microscope (SEM), transmission electron microscope (TEM), high resolution transmissionelectron microscope (HRTEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) andconfocal laser scanning microscopy (CLSM). It was found that inorganic semiconductor CdS and organicsemiconductor OPV3 homogeneously distributed into the hybrid nanorod. Fluorescence spectroscopy wasalso used to characterize the optical property of the CdS-OPV3 nanorods. And such prepared CdS-OPV3homogeneous hybrid nanorods exhibit different optical properties compared with individual component CdSnanorods and OPV3 nanorods. The as-prepared new kind of homogeneous hybrid organic/inorganicsemiconductor nanorods are potential candidates for nanoscale electronic and photonic devices.

Introduction

It is significant in materials science to create nanostructuresthat combine the optical properties, electrical properties andmechanical flexibility of organic semiconductors with the highstability of inorganic semiconductors.1 As interesting nano-structures, hybrid nanocomposites2 are able to control manyphysical properties and tune size and shape of nanostructures.Many new concepts for controlling synthesis hybrid nanocom-posites have been studied, such as metal-metal junctions,3

metal-polymer junctions,4 inorganic semiconductor junctions,5

inorganic semiconductor-metal junctions6 and semiconductor-carbon nanotube junctions.7 However, how to combine organicsemiconductors and inorganic semiconductors for producing theuniformity of the size and shape on nanoscale with uniqueproperty is still a challenge. And there has been no report onone-dimensional homogeneous hybrid nanostructures composedof organic/inorganic semiconductor materials.

Oligo(phenylenevinylenes) (OPVs) as organic semiconductorshave become widely investigated over the past few years dueto their applicability to light emitting diodes, field effecttransistors and other devices.8 Meanwhile inorganic semicon-ductor CdS have also been proved to be excellent photoactivematerials and charge transmission materials in optoelectronicdevices.9 Interestingly, self-assembly hybrid structures of OPVsand CdSe nanocrystals as film devices have been fabricated.10

In this report we would like to describe the concept forcontrolling the growth of homogeneous hybrid nanorods com-posed of organic semiconductors and inorganic semiconductorsby template method.11 The growth depends on selecting use ofthe organic unit oligo(p-phenylenevinylene) (OPV3) (Figure 1a)in which two ester groups could benefit by being linked on thesurface of inorganic semiconductor CdS for forming of a typical

organic/inorganic hybrid interface with ordered arrays andproducing different optical properties compared with the singlecomponent nanorods.

The combination of organic/inorganic semiconductors con-tains a large amount of homogeneous hybrid interfaces betweenCdS and OPV3 molecules. The organic-inorganic hybridinterface in the system is able to make the material suitable forapplications on photovoltaic devices and light emitting devices.12

Importantly, in our experiment, blue-green and red fluorescentemissions are detected on a single hybrid nanorod that willenable us to exploit them for designing and building novelelectronic and photonic devices and other applications.

Experiment Method

Materials. Sublimed sulfur, dimethyl sulfoxide (DMSO),cadmium acetate, tetrachloroethylene and acetone were pur-chased from Beijing Chemical Reagent Corporation, China.Octylamine was purchased from Aldrich Corp. The AAOtemplates with porous diameter of 200 nm and a thickness of60 µm were purchased from Whatman Co. All of the reagentswere used as received.

Synthesis of CdS Nanocrystals. CdS nanocrystals weresynthesized according to a modified literature method.13 Briefly,0.25 g of sublimed sulfur (0.007812 mol) was dissolved in 200mL of dimethyl sulfoxide (DMSO) at 100 °C for about 1 h.After that this solution was heated to 150 °C. Then, a preheatedsolution of 2.5 g of cadmium acetate (9.398mmol) in 200 mLof DMSO was added. The solution became lemon yellow afterseveral minutes. The reaction solution was heated for about 1 hand then cooled to ambient temperature. CdS nanocrystals weredeposited at the bottom of the reaction flask by adding acetoneto the solution. Finally, CdS nanocrystal powders were obtainedafter centrifuging, washing and drying under vacuum at roomtemperature for about 2 h.

The synthesis of organic semiconductor oligo(p-phenylenevi-nylene) (OPV3) was reported in our previous article.14

* Corresponding authors. E-mail: ylli@iccas.ac.cn; liuhb@iccas.ac.cn.† Institute of Chemistry, Chinese Academy of Sciences.‡ Graduate School of Chinese Academy of Sciences.

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10.1021/jp800456c CCC: $40.75 2008 American Chemical SocietyPublished on Web 05/10/2008

Fabrication of CdS-OPV3 Hybrid Nanorods. The CdS-OPV3 hybrid nanorods were fabricated by a simple templatemethod under the assistance of vacuum. Before the synthesisof CdS-OPV3 nanorods, 1 wt% CdS colloid was first preparedby adding 0.1 g of CdS nanocrystals (Figure 1b) and 1 mL ofoctylamine to 9 mL of tetrachloroethylene. Then, 11 mg ofOPV3 (Figure 1a) was added to 2 mL of 1 wt% CdS colloid.In order to acquire a homogeneous CdS-OPV3 solution, themixture was kept in a flask at 60 °C for about 10 min.

The whole synthetic process of CdS-OPV3 nanorods isbriefly illustrated in a scheme (Figure 1c). A porous AAOmembrane was immersed into the transparent CdS-OPV3mixture in a flask under reduced pressure. The mixture wasexposed to the atmosphere after being kept under reducedpressure for about 5-8 min. And the solution was poured intothe pores of the membrane instantly owing to the pressuredifferentiation.15 Take out and dry the membrane immediately.After repeating the process above three times, CdS-OPV3nanorods embedded into the channels of the AAO membranewere acquired. Finally, the AAO template was selectively etchedby NaOH solution (3 M) and cleaned under the assistance ofultrasonic cleaning machine for later analysis.

In order to investigate the different optical properties betweenhybrid nanorods and the individual component nanorods,individual component CdS nanorods and OPV3 nanorods werealso fabricated via the above process.

Characterization. The morphologies of the CdS-OPV3hybrid nanorods and energy-dispersive X-ray spectroscopy(EDS) were observed on a JEOL JSM 6700F field-emissionscanning electron microscope, at an accelerating voltage of 15

kV. Transmission electron microscopy (TEM) measurementsof CdS nanocrystals were conducted with a JEOL 1011transmission electron microscope operating at an acceleratingvoltage of 100 keV. The TEM, HRTEM measurements andselective area electron diffraction pattern (SAED) of theCdS-OPV3 hybrid nanorods were taken with a JEOL 2010transmission electron microscope using an accelerating voltageof 200 keV. CLSM images were acquired with a WITecCMR200 in the confocal Raman spectra mode. A picosecondpulse laser (405 nm, 135 W, 20 MHz, PIL040F optical head,Advanced Laser Diode Systems, Germany) was used forexcitation. The XRD pattern was recorded with a Japan RigakuD/max-2500 rotation anode X-ray diffractometer equipped withgraphite monochromatized Cu KR radiation (k ) 1.5418 Å),employing a scanning rate of 8° min-1 in the 2θ range from20° to 60°. The UV-vis spectra were recorded on a HitachiU-3010 spectrophotometer, and the fluorescence spectra wererecorded in Hitachi Model F-4500 FL spectrophotometer withan excitation wavelength at 405 nm.

Result and Discussion

The characterizations of the CdS nanocrystals which havebeen used to fabricate the hybrid CdS-OPV3 nanorods areshown in Figure 2. From the TEM image (inset in Figure 2),the average diameter of the CdS nancrystals appears to be about5 nm. The absorption onset of 490 nm in the UV-vis absorptionspectrum (blue line in Figure 2) confirms that the average sizeof CdS nanocrystals is 5 nm.16 The fluorescence spectrum (redline in Figure 2) of CdS colloid shows two fluorescence

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.

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emissions: one emission peak is centered at 470 nm; the otherranges from 530 to 790 nm. The fluorescence emission peakcentered at 470 nm is the band gap emission of CdS nanocrystalswith size ranging from 5 to 14 nm. Compared with the band-edge emission of bulk CdS (ca. 510 nm), the emission peak isabout 40 nm blue-shifted because of the quantum confinementeffect of CdS nanocrystals. The other strong broad fluorescenceemission ranging from 530 to 790 nm is attributed to the surface-defect emission which is caused by the surface state of CdSnanocrystals such as sulfur vacancies and/or sulfur danglingbonds.17

The morphology of the combined CdS-OPV3 nanorods wasobserved by scanning electron microscopy (SEM). Figure 3Adepicts that the alumina template filled with the CdS-OPV3nanorods presents a uniform and beautiful bright yellow. Figure3B shows a large area of the CdS-OPV3 nanorods in a lowermagnification. The large amount of nanorods indicated the highfilling density of the membrane. The nanorods shown in Figure3C exhibit the same orientation and monodispersed diametersof 200-150 nm and length of 2-5 µm. The results are in goodagreement with the pore diameter of the template. Figure 3Dshows the top of two typical CdS-OPV3 nanorods. Thesuccessful hybrids of CdS and OPV3 can be demonstrated bythe element signature of C, S and Cd in the energy-dispersiveX-microanalysis (Figure 3E). Quantitative analysis indicates thatthe atomic ratio of Cd and S is about 1:1. The Si peak of thespectrum results from the Si substrate. The signature of Pt isthe contribution of Pt layer covered for the SEM analysis. TheC and O atom signals are detected in the analysis, indicatingthat the nanorods contain the component of OPV3. On the basisof quantitative EDS analysis of hybrid nanorods (Figure 3E),the ratio of CdS and OPV3 in the hybrid nanorods is about13:1. To investigate the distribution of CdS nanocrystals andOPV3 in the hybrid nanorods, EDX element maps werecollected by scanning the whole CdS-OPV3 hybrid nanorod.The real scanning area on the single CdS-OPV3 hybrid nanorodwas labeled by the yellow square in Figure 3F. The respectiveCd, S, C and O element maps directly proved the homogeneoushybrid of CdS nanocrystals and OPV3 molecules in theCdS-OPV3 hybrid nanorod. EDX element maps of othernanorods investigated also exhibit similar results. The highlyhomogeneous hybrid CdS-OPV3 nanorods would be an ef-ficient candidate in the area of energy conversion.

Further structure information and characterization of theCdS-OPV3 nanorods were performed by transmission electronmicroscopy (TEM) in Figure 4. Figure 4A depicts a few hybridnanorods of the CdS-OPV3 with diameter of 250 nm and lengthof 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 displaysthe end of an independent hybrid nanorod. A HRTEM image(Figure 4C) reveals the exact structure of the nanorod: the single-crystalline nanoparticles of CdS embedded into OPV3 homo-geneously. The two-dimensional lattice part (indicated by thewhite circle) is CdS nanocrystal. And the amorphous part(indicated by the black circle) is OPV3. The HRTEM image asshown (Figure 4D) supplies precise information on the interfacebetween CdS single-crystal nanoparticle and OPV3. The latticespacing around 0.204 nm observed in this image agrees wellwith interplanar distance of the (110) direction parallel in thewurtzite phase of CdS. And the wurtzite structure of CdSnanocrystals can also be proved by XRD (Figure 5). Thetransition layer of CdS single-crystal nanoparticle and amor-phous OPV3, which is about 1-2 nm, also has been clearlyexhibited (the area between two black curves in Figure 4D).

The XRD patterns for CdS-OPV3 hybrid nanorods and CdSnanocrystals are shown in Figure 5. The hybrid nanorods andCdS nanocrystals have the same diffraction patterns. And allof those peaks can be assigned to (100), (002), (101), (110),(103) and (112) planes of wurtzite structure CdS.18 Thisinformation corresponding to the conclusion of HRTEM indi-

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

Figure 3. (A) Photograph of the alumina template filled withCdS-OPV3 nanorods. Scanning electron microscope (SEM) imagesof hybrid CdS-OPV3 nanorods: (B) large area of CdS-OPV3 nanorodsafter partial removal of the template; (C) well-ordered CdS-OPV3nanorod arrays under a higher magnification; (D) typical image ofCdS-OPV3 nanorods. (E) Energy-dispersive X-microanalysis spectrum(EDS) of the CdS-OPV3 hybrid nanorods. (F) Element mapping of asingle CdS-OPV3 hybrid nanorod.

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cates that the hybrid nanorod is composed of wurtzite structureCdS nanocrystals.

It is known that nanostructures fabricated by the AAOtemplate wetting method are usually nanotubes.19 In a typicalwetting process, a thin film of solution (or polymer melts) wettedand covered the pore walls of the AAO membrane in the initialstage since the cohesive driving force for complete filling ismuch weaker than the adhesive force. And nanotube formedwhen evaporation of solvent or cooling led to crystallization orvitrification.20 However, in our experiments, the formation ofCdS-OPV3 nanorods experienced a different process. Whenthe AAO template was immersed into the transparent CdS-OPV3colloid, the poor imbibitions of tetrachloroethylene preventedthe thin film formation on the walls of AAO template pores.The CdS-OPV3 colloid was immediately driven into AAOpores by the pressure differentiation. Meanwhile, during theabove process, the homogeneous assembly colloid of CdS-OPV3as a whole had been poured into the AAO template pores. Thusthe CdS-OPV3 nanorods prepared by this method show a highhomogeneity across the whole nanorod.

Based on the fluorescent properties of the two componentsof CdS and OPV3, confocal laser scanning microscopy (CLSM)was used to confirm the structure of the hybrid nanorods. Figure6(A, B, C) shows the fluorescent images of a single CdS-OPV3nanorod (λex ) 405 nm). Figure 6A shows the fluorescent image

of the single hybrid nanorod by collecting fluorescent emissionsfrom 366 to 929 nm. Within the resolution of the opticalmicroscope, the nanorod represents a rather uniform intensityof fluorescence. Figure 6B displays that the fluorescent imageformed by accepting emissions from 470 to 490 nm presentseven the same morphology as Figure 6A. The fluorescenceemissions of CdS (bandgap emission) and OPV3 overlap in thisarea, but the enhanced fluorescence intensity in both thefluorescent image and the fluorescence spectrum demonstratethe contribution of OPV3. So this image confirms that OPV3distributed into the CdS-OPV3 nanorod uniformly. Figure 6Cshows that the fluorescent image formed by accepting thesurface-trap emissions of CdS (557-651 nm). This fluorescentimage directly depicts that the surface-trap emission of CdS at557-651 nm is much weaker than the emissions ranging from470 to 490 nm.21 In fact, the uniform dispersion of CdSnanocrystals in the hybrid nanorod is also confirmed. Thesuccessful fabrication of CdS-OPV3 homogeneous hybridnanorod was rigorously confirmed by the CLSM investigation.

We investigated the different optical properties of indi-vidual component OPV3 nanorods, CdS nanorods andCdS-OPV3 hybrid nanorods. Figure 6D shows the CLSMfluorescence spectra of CdS nanorods, OPV3 nanorods andhybrid nanorods. As shown in Figure 6D, one of the emissionpeaks of the hybrid nanorods (red curve) centers at 490 nm,which is different from individual component nanorods ofCdS (479 nm) and OPV3 (503 and 528 nm); another is abroad emission peak which ranges from 550 to 700 nm. Theemission of hybrid nanorods is stronger than that of CdSnanorods (black curve), and the fluorescence emission ofOPV3 is decreased by CdS nanocrystals through the largeamount of homogeneous hybrid interfaces in the hybridnanorods. The different optical properties of single componentCdS nanorods, OPV3 nanorods and the hybrid nanorods arealso investigated by fluorescence equipment in Figure 7.Figure 7A shows the normalized fluorescence spectra ofOPV3 nanorods, CdS nanorods and CdS-OPV3 hybridnanorods. Due to the large amount homogeneous hybridinterfaces of CdS and OPV3, the characteristic emission ofCdS-OPV3 hybrid nanorods (red curve) centers at 499 nm,which is between the peaks of single OPV3 nanorods (blue

Figure 4. Transmission electron microscope (TEM) images of theCdS-OPV3 hybrid nanorods: (A) TEM image of a few CdS-OPV3nanorods. The inset in (A) is the selective area electron diffractionpattern (SAED) taken from the nanorods; (B) TEM image of a typicalCdS-OPV3 hybrid nanorod; (C) HRTEM image of the CdS-OPV3hybrid nanorod; (D) HRTEM image of CdS-OPV3 interface under ahigher magnification.

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

Figure 6. CLSM images (λex ) 405 nm) of a single CdS-OPV3hybrid nanorod collected (A) from 366 to 929 nm; (B) from 470 to490 nm; (C) from 557 nm to 651nm. (D) The CLSM fluorescencespectrum of OPV3 nanorods, CdS nanorods and CdS-OPV3 hybridnanorods.

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curve) and single CdS nanorods (black curve). Under theexcitation wavelength of 405 nm, the emission peak of OPV3nanorods (blue curve) appears at 510 nm, the bandgapemission of CdS nanorods is at the position of 492 nm (blackcurve) and the surface-trap emission of CdS nanorods rangesfrom 550 to 700 nm. However the surface-trap emission ofCdS nanocrystals in the hybrid nanorods is too weak to beseen in contrast to the band gap emission in the fluorescencespectra. Figure 7B shows the fluorescence spectra of OPV3nanorods, CdS nanorods and CdS-OPV3 hybrid nanorods.The inset of Figure 7B is the fluorescence spectrum of CdSnanorods (black curve), which is too weak to be seen in onediagram with the fluorescence spectrum of OPV3 nanorods(blue curve). The fluorescence spectra display that theemission of OPV3 decreased greatly in the hybrid nanorods.It is believed that the emission of OPV3 was quenched byCdS nanocrystals. And the fluorescence quenching is indica-tive of possessing charge transfer between the two semicon-ductors that is a characteristic process in such a type ofinorganic/organic photovoltaic materials.22 The CLSM fluo-rescence spectra in Figure 6D and fluorescence spectra inFigure 7 demonstrate the obviously different optical propertyof the hybrid nanorods by comparing with the two individualcomponent nanorods. The results indicate that there are stronginteractions between the two components.

Conclusion

In summary, this work represents a typical fabrication ofhomogeneous composite nanorods based on the hybrid oforganic semiconductor and inorganic semiconductor. Thecombination of organic/inorganic semiconductor nanorodsexhibited a different optical property than the two individual

component nanorods, which may be caused by a large amountof homogeneous hybrid interfaces. The approach is ageneralized process that can be used to synthesize differenthybrid nanorods of organic semiconductors/inorganic semi-conductors with high uniformity of the size and shape, whichshould have outstanding potential in providing developmenthybrid structures of ordered nanoscale architectures forphotonic electronic and other devices.

Acknowledgment. This work was supported by the NationalNature Science Foundation of China (20531060 and 20571078)and the National Basic Research 973 Program of China (GrantNo. 2006CB932100 and 2005CB623602).

References and Notes

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

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