Morphology Control in the Vapor-Liquid-Solid Growth of SiCNanowires

Huatao Wang,† Zhipeng Xie,*,† Weiyou Yang,‡ Jiyu Fang,§ and Linan An*,§

State Key Laboratory of New Ceramic sand Fine Processing, Department of Materials Science andEngineering, Tsinghua UniVersity, Beijing 100084, China, Institute of Materials, Ningbo UniVersity ofTechnology, Ningbo 315016, China, and AdVanced Materials Processing and Analysis Center,UniVersity of Central Florida, Orlando 32816

ReceiVed March 16, 2008; ReVised Manuscript ReceiVed August 20, 2008

ABSTRACT: In this paper, we report a new technique to manipulate and control the morphology of vapor-liquid-solid (VLS) grownSiC nanowires by varying the pressure of the source species. We demonstrate that the diameter of the nanowires is strongly related to thepressure and pressure variation rate of the source species. SiC nanowires with Eiffel-tower shape, spindle shape, and modulated diametersand periods have been synthesized. In principle, the technique is applicable to other material systems.

Introduction. Semiconducting nanowires are critical buildingblocks in the “bottom-up” fabrication of optoelectronics, quantumcomputing, and sensors.1 Conventionally, the nanowires aresynthesized via a vapor-liquid-solid (VLS) process,2 which wasfirst proposed by Wagner and co-workers to account for the growthof silicon wires.3 For device applications, it is important to achievethe control growth of nanowires. Previous studies of VLS growthof nanowires were primarily focused on controlling their diametersby altering the size of catalytic droplets or synthesizing heteroge-neous structures by altering tip chemistry.4,5 In these previousstudies efforts have been devoted to synthesize nanowires withconstant diameters. However, for some applications nanowires withvarying morphologies are desired. For example, nanowires withspindle or lathe shapes are useful building blocks for nanoelectro-mechanical systems (NEMS). The periodic variations in nanowiresdiameters could be used as bench-marks for precisely positioningthem in device fabrication.

Silicon carbide (SiC) is one of the important wide bandgapsemiconductors with a band gap from 2.8 to 3.4 eV, depending onits crystal structure. It has been shown that silicon carbide possessesexcellent thermo-mechanical properties, including high strength andstiffness, high-temperature stability, corrosion resistance, and highthermal conductivity, making it suitable for applications in high-temperature/high-power electronics, short-wavelength optics, andnanoelectromechanical systems.6 It is expected that as the continu-ing development of nanotechnology, SiC may find widespreadapplications in many fields.7 Procedures for growing one-dimensional SiC nanostructures with various shapes, such as nano-wires, nanorods, nanobelts, nanotubes, nanocables, nanosprings, andnanospheres, have been developed successfully.8-17 However, thetechnique that can be used to synthesize SiC nanowires with variedmorphologies in a controlled manner has not been reported yet.

In this communication, we report for the first time a novel methodto synthesize SiC nanowires with varied morphologies in acontrolled manner by simply altering the pressure of the sourcespecies. We demonstrate the feasibility of the technique bysynthesizing SiC Eiffel-towers, spindles and nanowires withmodulated morphologies. The technique will add significant flex-ibility in the control growth of SiC nanowires.

Experimental Section

Synthesis. The SiC nanowires were synthesized using com-mercially available polysilazane (Ceraset, Kion, USA) as the sourcematerial. The polysilazane was first solidified by heat-treatment at260 °C for 30 min and then ground into powders. The powderswere then placed at the bottom of a high-purity alumina crucible.A graphic sheet of ∼2 mm thick was used as the substrate. Thegraphic sheet was first immersed in ethanol solution of Fe(NO3)3

with 0.2 mol/L concentration. After drying naturally, the graphicsubstrate was vertically placed in the alumina crucible on the topof the polysilazane powders. The crucible with the source materialand the substrate was heat-treated in graphite-heater furnace at 1550°C in still Ar atmosphere. During the heat-treatment, Ar pressurewas altered according the schedule described in Figures 3, 4 and 5for synthesizing the SiC nanowires with different morphologies.A previous study reported that the polysilazane decomposed intoSiCN(O) amorphous ceramics when heated to 1000 °C.18 Furtherincreasing the heat-treatment temperature resulted in the crystal-lization of the SiCN(O) to form SiC, Si3N4 and graphic phases,accompanied by releasing SiO and CO gases.19 These gaseousspecies reacted with Fe within Fe(NO3)3 to form liquid Fe-Si-Calloys, which reacted with the gaseous species further to lead tothe VLS growth of the SiC nanowires.

Characterization. The obtained nanowires were analyzed usingscanning electron microscopy (SEM, LEO 1530, Gemini, Germany),and high-resolution transmission electron microscopy (HRTEM,JEML-2011, JEOL, Tokyo, Japan). The TEM samples wereprepared by ultrasonically dispersing the nanowires in ethanolsolution and drop on Cu grids coated with carbon film.

Results and Discussion. To set a baseline for investigating theeffect of source pressures on the growth of SiC nanowires, we firstprepare SiC nanowires at a constant pressure of the source speciesby setting Ar pressure to 0.11 MPa. Figure 1 shows the productobtained after 10 min synthesis time. The product has a cone-shapewith a catalytic droplet at the top of its thick tip (Figure 1a,b).Selected area electron diffraction (SAED) analysis of the productsuggests that the cone is 6H-SiC (Figure 1c). The formation of theSiC phase is due to its stability at this processing condition.20,21

The SAED pattern reveals that the cone grows along the [0001]direction. High-resolution transmission electron microscopy(HRTEM) observation shows two sets of fringes with the d-spacesof 0.25 and 0.22 nm, corresponding to the {101j2} and {1j104}planes of 6H-SiC, respectively (Figure 1d). The HRTEM resultconfirms that the cone grows along the [0001] direction. It isinteresting for us to see the 6H-SiC grows into a cone shape ratherthan a wire shape. This can be explained using the growth

* Corresponding author. E-mail: xzp@mail.tsinghua.edu.cn (Z.X.) andlan@mail.ucf.edu (L.A.).

† Tsinghua University.‡ Ningbo University of Technology.§ University of Central Florida.

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10.1021/cg8002756 CCC: $40.75 2008 American Chemical SocietyPublished on Web 10/11/2008

mechanism shown in Figure 1e. First, the SiC seeds are nucleatedat the interface between the catalytic droplet and the graphicsubstrate via a heterogeneous nucleation process. Since the smallsize of the nuclei, the large contact angle (labeled as θ in Figure1e) between the nuclei and the catalytic droplet is expected togenerate an outward force on the wire.22 Thereby, at the early stagethe droplet cannot effectively confine the lateral growth of the wire.

As the growth of the nanowire along the lateral direction, the contactarea between the wire and the catalytic droplet increases, leadingto the decrease in the contact angle and in the outward force.Eventually, the lateral growth is effectively confined by the catalyticdroplet, leading to the formation of nanowires with a relativelyconstant diameter (Figure 2).

Now, we turn to the preparation of SiC nanowires with variedmorphologies by altering the pressure of the source species viachanging Ar pressure during synthesis. The nanowires with anEiffel-tower shape are synthesized using the process detailed inFigure 3a. The Eiffel-tower shaped nanowires consist of a cone,an Eiffel-tower base, and an Eiffel-tower tip (Figure 3b,c). High-magnification SEM image shows that the Eiffel-tower tip is about1 µm long with a diameter of ∼20 nm (inset in Figure 3b). A SAEDanalysis (Figure 3e) recorded from Figure 3d reveals that the tower-top tip is also 6H-SiC and grows along the [0001] direction. The

Figure 1. (a) SEM image of SiC cones obtained at 1550 °C for 10min in ultrahigh purity Ar of 0.11 MPa. (b) TEM image of a part of aSiC cone. (c) SAED pattern taken from the area as marked by the squarein (b). (d) HRTEM image taken from the area marked by the square in(b). (e) Schematic showing the growth mechanism of the SiC cones.

Figure 2. (a) SEM image of the SiC nanowires obtained at 1550 °Cfor 35 min in ultrahigh purity Ar of 0.11 MPa. (b) High-magnificationSEM image of the SiC nanowires. The images reveal the nanowireswith relatively constant diameter following initial cones.

Figure 3. (a) Schematic showing a three-step processing schedule. (b)High-magnification SEM image of an Eiffel-tower shaped SiC nano-wire; the inset is an enlarged image of the tip area (the scale barcorresponds to 10 nm). (c) SEM image of Eiffel-tower shaped SiCnanowires. (d) TEM image of a nanowire. (e) and (f) SAED patternand HRTEM image, respectively, taken from the area marked by thesquare in (d).

Figure 4. (a) Processing schedule showing the change in pressure as afunction of time. (b) SEM image of SiC spindles obtained using theprocessing schedule described in (a). (c) High-magnification SEM imageof the SiC spindles.

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SEAD patterns taken from the cone and the tower base are identicalwith that from the tower tip, suggesting the whole nanowire is asingle crystal. The HRTEM image (Figure 3f) taken from the tower-tip reveals that it has a perfect crystal core surrounded by a thinamorphous shell with a thickness of ∼1 nm on its surface.

The unique Eiffel-tower shape is resulted from the specialsynthesis processing (Figure 3a). In this synthesis process shownin Figure 3a, Ar pressure was first remained at 0.11 MPa for 6 minand then increased to 0.4 MPa within 1 min and kept at the pressurefor 4 min. Finally, Ar pressure is decreased to 0.11 MPa from 0.4MPa within 1 min and kept at 0.11 MPa for 8 min. Since the partialpressures of Si/C-containing species remain constant in the firsttwo steps, thus the cone-shaped SiC nanowires are formed, similarto the growth of SiC cones as shown in Figure 1. However, in thefinal step, Ar pressure is quickly decreased from 0.4 MPa to 0.11MPa. The release of Ar can bring Si/C-containing species with it.The partial pressure of the Si/C-containing species is suddenlydecreased to ∼27.5% of its original value within 1 min. The quickdecrease in the pressure of the Si/C-containing species leads to lesssource materials which supply the catalytic droplet for the nanowiregrowth. The quick shrinkage in the diameter of the nanowire dueto the loss of source materials results in the formation of a towerbase. The long and shape tower-tip is formed in the subsequentgrowth for longer synthesis time. It is obviously that the final stepis critical for the success formation of the Eiffel-tower shapednanowires, in which the partial pressure of the source species isaltered. The Eiffel-tower shaped SiC nanowire with a long and smalltip could be useful for field-emission applications, given theexcellent emission behavior of SiC.23

Figure 4b and 4c present a spindle-shaped nanowire synthesizedusing the process shown in Figure 4a. This process is similar tothat used to prepare the Eiffel-tower shaped nanowires (Figure 3a),except that the time of releasing Ar pressure increases from 1 to 2min. The increase of the released time causes the decrease of thereleasing rate of source species by a factor of 2. As can be seen,the small change on the synthesis conditions can lead to a significantchange in the morphologies of the resultant nanowires. First, thediameter of the spindle shrinks at a lower rate during gas releasing,as compared to that in the Eiffel-towers. This may be due to theslow depleting of Si/C-containing species. Second, there is no sharptip formed for the spindles even when the same synthesis time asthe Eiffel-towers is used. Instead, the top part of the spindle growsinto a cone shape. This result, together with that shown in Figure3, suggests that the morphology of SiC nanowires can be well-controlled by simple control of the gas releasing rate.

To further explore the capability of controlling the morphologyof SiC nanowires by altering the pressure of source species, wesynthesized SiC nanowires by repeating pressure increasing/decreasing cycles. Figure 5b shows an SEM image of modulated

nanowires synthesized using three pressure cycles shown in Figure5a. The modulated nanowires consist of regularly spaced “notches”(Figure 5c). The modulation is a direct result of the pressure cycles.The first pressure cycle is similar to that present in Figure 4a,leading to the formation of a spindle shaped section. After the firstpressure drop, a longer synthesis time is allowed (the annealingtime is 31 min between first pressure drop to the second pressuredrop, while that is 7 min in Figure 4a), which leads to an increaseof the wire diameter due to the pressure recovery. At an even longersynthesis time between the second and third pressure drops, asection of nanowires with a constant diameter is obtained (Figure5c). It is clear that we can introduce notches at desired positionsalong the SiC nanowires by controlling the pressure cycles, whichis critical in developing their applications as lathe and beams withcontrolled periods.

Conclusions. In summary, we report a new technique tomanipulate and control the morphology of VLS grown SiCnanowires by varying the pressure of the source species. Eiffel-tower, spindle and modulated nanowires have been synthesized bythe technique. While cone or tip-shaped SiC nanowires have beensynthesized previously,24,25 the technique presented here allows usto shape SiC nanowires in a controlled manner. More important,the current technique can be used to synthesize modulated SiCnanowires. We believe that the technique could be applied to othermaterials as well since the fundamentals involved in the processingdo not have any special requirements for SiC.

Acknowledgment. The work is financially supported by theNational Science Foundation of China (No. 50372031), the two-based projects of NSFC (No. 50540420104) and the SpecializedResearch Foundation for the Doctoral program of Higher Education(No. 20050003004).

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