From CdTe Nanoparticles Precoated on Silicon Substrate to LongNanowires and Nanoribbons: Oriented Attachment ControlledGrowth

Xuebo Cao,* Xianmei Lan, Yang Guo, and Cui Zhao

Key Lab of Organic Synthesis of Jiangsu ProVince and Department of Chemistry, Suzhou UniVersity,Suzhou, Jiangsu 215123, P. R. China

ReceiVed July 18, 2007; ReVised Manuscript ReceiVed October 29, 2007

ABSTRACT: This manuscript describes a simple, environmentally friendly strategy for the rapid and large-scale growth of ultralongnanowires and nanoribbons of wurtzite CdTe. The nanowires and nanoribbons were prepared through the direct hydrothermal treatmentof CdTe nanoparticles precoated on ⟨100⟩ Czochralski silicon, which did not involve complicated reactions. The well-crystallinenanowires and nanoribbons were grown along the [102] direction and were up to 100 µm long. The growth of the nanowires andnanoribbons was dominated by the mechanism of oriented attachment, which was clarified through the tracing of the temporalevolution of CdTe nanoparticles coated on the silicon substrate in the process of hydrothermal treatment. Furthermore, the proposedstrategy was also effective in the preparation of anisotropic nanostructures of other II-IV group compounds (e.g., ZnO and CdSe).

1. Introduction

Because of its optimum band gap (Eg ≈ 1.5 eV), whichmatches the solar spectrum well, CdTe has drawn much attentionin recent years.1 Especially because its size is in the order ofseveral nanometers, CdTe exhibits strong quantum confinementeffects and enhanced physical properties (e.g., high photolu-minescence and photo-to-electric conversion efficiencies).2,3

Consequently, CdTe nanocrystals have many important technicalapplications in photovoltaic devices,4 solar cells,5,6 and biola-beling and detection.7,8 Currently, numerous efforts are devotedto the controlled preparation of nanostructures of CdTe withthe desired shapes and sizes.9 The most extensively studiedCdTe nanostructures should be quantum dots (QDs), which areconsidered promising fluorescent probes to replace organic dyemolecules.7 Besides QDs, one-dimensional (1D) CdTe nano-structures with a high aspect ratio such as wires,10–17 tubes,18

or ribbons are also particularly attractive because they haveshape-dependent physical properties and can serve as veryinteresting building blocks for preparing unique superstructuresthat cannot be realized with spherical or short rod-shapednanocrystals.

Compared to diverse synthetic strategies for the preparationof CdTe QDs, the methods for preparing CdTe nanowires arerather limited,10–17 and the preparation of CdTe nanoribbonshas not been reported so far. For instance, Xu et al. preparedCdTe nanowires by the restricted growth in the pores of anodicaluminum oxides10 and Niu et al. prepared core/shell CdTenanowires in the micelles formed by amphiphilic triblockcopolymers.14 More recently, Kuno and co-workers havesynthesized straight and branched CdTe nanowires via asolution-liquid-solid route, by using low-melting Au or Binanoparticles as the catalyst to induce the preferential growth.17

Besides these normal approaches based on chemical reactionsbetween metal and chalcogen precursors, Kotov and co-workershave developed a novel and simple self-organization growthroute to prepare uniform CdTe nanowires. They first removedpartial capping agents on the surface of quantum-sized CdTeparticles and then dispersed the particles into the solution. After

several days, the particles were aggregated spontaneously andfused into nanowires with a high aspect ratio.11,19 The drivingforce for the aggregation is the elimination of the surface andthe reduction of the total energy of the system.20,21 In thecoalescence process of the aggregated particles, the intrinsicanisotropic structure of wurtzite CdTe can act as a naturaltemplate to direct the preferential growth, which eventuallyresults in the formation of nanowires.

Generally, the homogeneous nucleation and growth of crystalsin the solution needs to overcome a higher kinetic energy barrierthan that of the same process taking place on a solid substrate.22,23

Consequently, the anisotropic growth of the nanocrystals canproceed more easily and quickly if it is performed on a substrate.With this principle, Vayssieres et al. have grown 1D nanostruc-tures of diverse materials (e.g., SnO2,22 ZnO,24 and Fe2O3

25)by introducing polycrystalline F-SnO2 glass substrate, siliconwafer, or ITO-coated polyester substrate into the system. Thesestudies give important indications that the self-organizationgrowth of anisotropic CdTe nanocrystals may be facilitated bythe phase transition of cubic CdTe nanoparticles predepositedon a solid substrate under appropriate conditions. Herein, toverify the validity of this presumption, we study the growthbehavior of thioglycolic acid (TGA)-stabilized CdTe nanopar-ticles coated on the surface of a ⟨100⟩ Czochralski silicon waferunder hydrothermal conditions. It is found that uniform CdTenanowires and nanoribbons with lengths up to 100 µm aresuccessfully prepared in a short period (12 h). Becausefunctional nanodevices are usually substrate-related, the directgrowth of CdTe nanowires and nanoribbons on silicon substratecan facilitate their integration into nanodevices. Furthermore,the proposed strategy of the hydrothermal treatment of precur-sors deposited on a solid substrate can generally be used in thepreparation of anisotropic nanostructures of other materials, suchas ZnO and CdSe.

2. Experimental Section

Preparation of CdTe Nanowires. All the reagents (analytical grade)were purchased from Shanghai Chemical Reagent Corp. and used asreceived. Before being used, silicon substrates (size: 5 mm × 5 mm)were cleaned in acetone by ultrasound. CdTe nanoparticles wereprepared according to the method presented in a previous report.26

* Author to whom correspondence should be addressed. Tel: 86-512-65880323. Fax: 86-512-65880089. E-mail: xbcao@suda.edu.cn.

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10.1021/cg700665s CCC: $40.75 2008 American Chemical SocietyPublished on Web 01/01/2008

Briefly, 0.228 g of CdCl2 ·2.5H2O (1 mmol) and 0.11 g of TGA (1.2mmol) were dissolved in 50 mL of distilled water, and the pH of thesolution was adjusted to 11.0 with 1 mol/L NaOH solution. Then, afresh oxygen-free NaHTe solution was injected in accordance with astoichiometric ratio of Cd2+ ions under vigorous stirring. The resultingfaint yellow solution was refluxed until its color turned to a brightyellowish orange, suggesting the formation of quantum-sized CdTenanoparticles. To purify the CdTe nanoparticles, 5 mL of dilutedhydrochloric acid (0.1 mol/L) was added to the solution, and CdTenanoparticles were precipitated immediately. The solids were separatedfrom the solution by centrifugation at 2500 rpm and washed severaltimes with absolute alcohol before being redispersed in distilled water.Afterward, a drop of the aqueous solution of CdTe nanoparticles (0.01g of CdTe in 1 mL of deionized water) was spread on the surface ofa piece of ⟩100⟨ Czochralski silicon wafer (size: 5 mm × 5 mm). Afterthe system dried at room temperature, a layer of gray coating was left

on the surface of the substrate. The substrate was then placed in aTeflon-lined stainless-steel autoclave filled with water and heated at140 °C for 12 h. After cooling down to room temperature, the substratewas taken out, rinsed with distilled water, and dried in vacuum for thesubsequent characterizations.

Characterization. X-ray powder diffraction (XRD) patterns wererecorded on an X’Pert PRO SUPER rA rotation anode X-ray diffrac-tometer with Ni-filtered Cu KR radiation (λ ) 1.54178 Å). Transmissionelectron microscopy (TEM) images, high-resolution transmissionelectron microscopy (HRTEM) images, and selected-area electrondiffraction (SAED) patterns were taken at 200 kV with a JEOL-2010electron microscope. Field-emitting scanning electron microscopy(FESEM) images were taken with a Hitachi S-4700 microscope withan accelerating voltage of 15 kV.

3. Results and Discussion

Figure 1a shows the XRD pattern of the primitive CdTenanoparticles, in which all the reflections correspond to thoseof zinc blende CdTe (JCPDS file, No.75-2086). The width ofthe peaks is related to the small size of the nanoparticles. Theaverage diameter of the nanoparticles is calculated with theScherrer equation to be 4.2 nm. Figure 1b shows the TEM imageof the primitive CdTe nanoparticles, which reveals that theyare actually composed of very small particles. The statisticaldiameter of the nanoparticles is around 4.9 nm, which is in goodagreement with that calculated with the Scherrer equation.Because these small nanoparticles have a large surface area anda high surface energy that result from their quantum-scale sizes,they should be very active and are ideal candidates for studyingthe self-organization growth on a solid substrate. Figure 1cshows the FESEM image of CdTe nanoparticles deposited on

Figure 1. (a) XRD pattern of the primitive CdTe nanoparticles. (b) TEM image of the primitive CdTe nanoparticles. (c) FESEM image of the CdTenanoparticles coated on the silicon ⟨100⟩ Czochralski silicon wafer for the hydrothermal treatment. A dense film with a thickness of 2 µm coveredthe surface of the silicon substrate.

Figure 2. XRD pattern of the products that have been treatedhydrothermally at 140 °C for 12 h. CdTe has changed phase composi-tion from the initial cubic structure to the wurtzite structure.

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the surface of the ⟨100⟩ Czochralski silicon wafer for thehydrothermal treatment. For the preparation of the film, we usedthe CdTe nanoparticles precipitated from the solution withdiluted hydrochloric acid rather than those from the solutionwith alcohols (e.g., methanol) that was widely used to purifythe CdTe QDs. Indeed, the CdTe nanoparticles precipitated fromthe solution with alcohols keep a good water solubility andwould dissolve into the solution during the hydrothermaltreatment. The solubility of the CdTe nanoparticles precipitatedwith hydrochloric acid is decreased dramatically; hence, thenanoparticles can attach tightly to the surface of the siliconsubstrate. It can be seen in Figure 1c that a dense film of CdTenanoparticles with a thickness of 2 µm was formed on thesurface of the silicon substrate. Although there were many crackswithin the film, it was found that they did not negativelyinfluence the formation of long CdTe nanowires and nanorib-bons during the subsequent studies.

The products obtained through the hydrothermal treatmentof the CdTe nanoparticles precoated on the silicon wafer werefirst checked with the XRD technique, and the typical XRDpattern is shown in Figure 2. The pattern composed of somestrong and sharp reflections is totally different from that of the

primitive CdTe nanoparticles, demonstrating that the phasetransition has actually occurred within the hydrothermally treatedproducts, and the products assumed a good crystallinity. Byindexing the XRD pattern, the products were found to exist inthe form of wurtzite phase (space group: P63mc). The calculatedlattice constants a ) 4.324 Å and c ) 10.32 Å agree well withthe literature data (a ) 4.301 Å and c ) 10.25 Å, JCPDS file,No. 80-0090). In addition, when compared with the standarddiffraction pattern of wurtzite CdTe, the reflection from the (102)plane is found to be abnormally strong, suggesting that apreferential growth may occur along this direction.

Subsequently, FESEM and TEM were used to characterizethe morphology of the as-prepared wurtzite-structured CdTe.From the panoramic FESEM images (Figure 3a,b), it is foundthat the products are mainly composed of a mixture of smoothnanowires and nanoribbons, together with a small amount ofparticles (less than 10%). The fractions of nanowires andnanoribbons within the products are about 30% and 60%,respectively. Panels c and d of Figure 3 show the TEM imagesof nanowires and nanoribbons in the products, respectively. Theyreveal that these nanowires and nanoribbons are very uniformalong their longitudinal axis. The sizes of CdTe nanowires and

Figure 3. (a) and (b) FESEM images with various magnifications show that the products after the hydrothermal treatment are a mixture of nanowiresand nanoribbons, which have an extremely high aspect ratio. (c) TEM image of CdTe nanowires. (d) TEM image of CdTe nanoribbons. (e) HRTEMimage. (f) SAED pattern taken on a single CdTe nanoribbon. The nanowires have a similar lattice fringe and SAED pattern.

Nanowires and Nanoribbons from CdTe Nanoparticles Crystal Growth & Design, Vol. 8, No. 2, 2008 577

nanoribbons are also measured by using FESEM and TEM. Thediameter of the nanowires is about 30 nm and their lengths areup to 100 µm. The lengths of the nanoribbons are close to thoseof the nanowires, and the sizes for their narrow surfaces andwide surfaces are 30 and 100 nm, respectively. That is, the as-prepared wurtzite CdTe nanowires and nanoribbons have anextremely high aspect ratio. For applications of 1D nanostruc-tures in circuit design and fabrication of functional architectures,a high aspect ratio is a highly preferable property. In Kotov’sreports on CdTe nanowires grown from QDs dispersed in asolution, the diameter of the nanowires is close to the size ofQDs because the nanowires are formed through the attachmentof QDs one by one. By contrast, the lateral dimensions of theas-prepared 1D nanowires and nanoribbons herein are muchlarger than the size of the primitive nanoparticles. Consequently,the growth mechanism of the nanowires grown from nanopar-ticles deposited on a silicon substrate is somewhat different fromthat of nanowires grown from QDs dispersed in a solution, aswill be explained later.

To learn more about the structural details of the as-prepared1D nanostructures of CdTe, we used HRTEM and SAEDtechniques to measure them. Panels e and f of Figure 3 are therepresentative HRTEM image and SAED pattern taken on anindividual nanoribbon, respectively. The HRTEM image andSAED pattern obtained on the nanowires are the same as thoseof nanoribbons; hence, they are not presented here. Accordingto the HRTEM image (Figure 3e), the clearly resolved latticesdemonstrate that the nanoribbons have a single -crystalline

structure. The interplanar spacing of 3.05 Å corresponds to theseparation between the (102) lattice planes of the wurtzite CdTe,revealing that the nanoribbons are preferentially grown alongthe [102] direction. This result is consistent with the strongreflection of the (102) plane in the XRD pattern (Figure 2).Figure 3f shows the SAED pattern taken along the [010] zoneaxis, which can be indexed to CdTe in a wurtzite structure. TheSAED pattern also confirms that the nanoribbons are singlecrystalline and grown preferentially along the [102] direction.

Generally, anisotropic crystal growth starting from nanopar-ticles can be classified into two different mechanisms: aniso-tropic Ostwald ripening and oriented attachment. For the former,the small particles in the system tend to dissolve into thesolution, owing to their relatively large surface energy, and theions are reprecipitated onto the high-energy crystallographicplanes of the large particles,27–30 leading to the oriented growthalong these faces and the formation of nanoscale rods, wires,or other anisotropic structures. But for the latter, the anisotropicgrowth is realized through the attachment and coalescence ofparticles with their favorable crystallographic planes,31–35 whichis different from the process of dissolution and reprecipitationof ions dominated by the mechanism of anisotropic Ostwaldripening.27–30 Consequently, to accurately distinguish the growthof CdTe nanowires and nanoribbons in the present route, whichis controlled by either anisotropic Ostwald ripening or orientedattachment, it is necessary to trace the morphology evolutionof the CdTe nanoparticles in the process of hydrothermaltreatment.

Figure 4. Temporal evolution of CdTe nanowires and nanoribbons observed in the process of hydrothermal treatment. (a) and (b) Short nanowiresare found in the CdTe coating at the beginning stage of the hydrothermal treatment (t ) 4 h). The nanowires are formed through the linear aggregationof numerous small particles. (c) and (d) With increasing time, long wires and ribbons are the major components of the coating (t ) 6 h).

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Shown in Figure 4 are the observed intermediates at differentstages, which clearly display the growth process and the growthmechanism of the as-prepared nanowires and nanoribbons ofwurtzite-structured CdTe. Figure 4a,b shows the FESEM imagesof CdTe nanoparticles deposited on the silicon wafer that havebeen treated hydrothermally for 4 h. It can be seen that, incontrast to the film at the beginning (Figure 1c), both the surfaceand the interior of the film are covered with short nanowires atthis stage. By studying the FESEM image with a highmagnification, it is found that the nanowires are formed notthrough the normal point-initiated vectorial growth mode, butthrough the linear aggregation of numerous small particles(Figure 4b). With the increase of the time to 6 h, the majorcomponents of the film are found to be long wires or ribbons(Figure 4c). At this stage, the lengths of the wires and ribbonsare already close to that of the final products, whereas thesurfaces of the nanowires are still rough (Figure 4d). On thebasis of the above observations on the intermediates of linearlyaggregated particles (see Supporting Information for moredetails), it can be deduced that the formation of the CdTenanowires and nanoribbons is dominated by the orientedattachment mechanism.11,33–38 The driving force for the orientedattachment is the reduction of the surface energy, and thisprocess is thermodynamically favorable because the surfaceenergy is reduced substantially when the interfaces are elimi-nated. Generally, for a well-dispersed system, because the smallparticles are arranged one by one in the process of orientedattachment (Figure 5a), the diameter of the formed nanowiresis close to the size of the particles.11,19,38 But for an agglomer-ated system such as the film of CdTe nanoparticles in this study,as described in Figure 5b, the event can occur at any point ofthe particles surfaces, provided that the crystallographic condi-tions are favorable.33,36 Consequently, the size of our preparedCdTe nanowires and nanoribbons is obviously larger than thatof the initial single nanoparticles. In addition, in most reportson 1D wurtzite CdTe nanostructures, crystal growth tends tooccur along the c-axis because wurtzite CdTe is intrinsicallyanisotropic along this direction.11,27 However, herein, the growthof CdTe nanowires and nanoribbons is not along the normal[001] direction but along the infrequent [102] direction. Wesuppose that this is related to the TGA molecules adsorbed onthe CdTe nanoparticles, which can modify the oriented attach-ment mechanism by changing the surface energy and also bypreventing contact between the faces on which adsorption hasselectively occurred.33,36 An analogous case has been reportedfor titania nanowires grown with the oriented attachmentmechanism, the attached planes of which are not (001) with ahigh surface energy but (101) with a low energy.39

It should be noted that, for the successful preparation ofnanowires and nanoribbons of CdTe, the initial film coveringthe silicon substrate must be dense and thick. If a dilutedsolution (for example, 0.001 g of CdTe in 100 mL of water)were used to prepare the film, the surface of the substratewould be covered by sparse nanoparticles of CdTe, and thefinal products would also be irregular nanoparticles. Wepresumed that only a dense film of CdTe nanoparticles couldprovide enough starting materials for attachment and pref-erential growth. Sparse nanoparticles on the surface of thesubstrate would not satisfy the requirement for the attachmentof the nanoparticles.

Considering that other II-VI group compounds such asZnO and CdSe share structures and growth habits similar tothose of CdTe, their anisotropic nanostructures, theoreticallyspeaking, could also be obtained with the proposed strategy.Consequently, we have studied the growth behavior ofcolloidal ZnO particles and TGA-stabilized CdSe nanopar-ticles deposited on a silicon wafer under hydrothermalconditions (see Supporting Information for more details). Asexpected, the FESEM observations of the products grownon the substrate reveal that they actually feature anisotropicprofiles. For CdSe, the products are mainly composed ofshuttle-shaped nanocrystals. For ZnO, the morphology of theproducts is also shuttle-shaped, but the nanocrystals arearranged rather regularly on the surface of the substrate.Consequently, these studies demonstrate that our proposedstrategy is rather general in the rapid growth of 1D nano-structures of II-VI group semiconductors.

4. Conclusions

In summary, we have proposed a feasible strategy to preparehigh-quality CdTe nanowires and nanoribbons through thehydrothermal treatment of CdTe nanoparticles deposited on asilicon substrate. In contrast to results presented in previousreports, this method is superior in simplicity, rapidity, environ-mental benefit, and in obtaining nanowires and nanoribbons withan extremely high aspect ratio (up to 100 µm). The observationson the temporal evolution of nanoparticles coating on the siliconsubstrate reveal that oriented attachment growth is the dominat-ing mechanism in the formation of the nanowires and nanor-ibbons. It is reasonable to expect this simple method to beextended to systems of more materials. Such studies are inprogress.

Acknowledgment. This work was financially supported bythe Key Lab of Organic Synthesis of Jiangsu Province (P. R.China), the Education Department of Jiangsu Province, and theNational Natural Science Foundation of China (20601020).

Supporting Information Available: TEM image of CdTe coatingtreated hydrothermally for 2 h; FESEM image of CdTe coating treatedhydrothermally for 8 h. FESEM images of anisotropic ZnO and CdSenanostructures prepared with the proposed strategy. This informationis available free of charge via the Internet at http://pubs.acs.org.

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