Journal of the Korean Physical Society, Vol. 50, No. 6, June 2007, pp. 1799∼1802

Synthesis, Structural Characterization and PhotoluminescenceProperties of SiOx Nanowires Prepared Using a Palladium Catalyst

Hyoun Woo Kim,∗ Seung Hyun Shim, Jong Woo Lee, Chongmu Lee,Hae Jin Hwang, Sung-Yoon Chung, Hyung-Sun Kim and Sun Keun Hwang

School of Materials Science and Engineering, Inha University, Incheon 402-751

Geun Young Yeom, Nae-Eung Lee and Ji-Beom Yoo

School of Metallurgy and Materials Engineering, Sungkyunkwan University, Suwon 440-746

Young-Chang Joo, Hyeong Joon Kim and Euijoon Yoon

School of Materials Science and Engineering and Inter-UniversitySemiconductor Research Center, Seoul National University, Seoul 151-742

(Received 3 September 2006)

We report the production of SiOx nanowires by thermal heating of Pd-coated Si substrates.Scanning electron microscope images indicated that the nanowires had diameters in the range of 15– 100 nm. X-ray diffraction, selected area electron diffraction, and energy dispersive X-ray analysesrevealed that the nanowires were amorphous and comprised of silicon oxide only. Photoluminescencemeasurements showed that the SiOx nanowires had an emission band in the blue region. We discussa possible growth mechanism for the SiOx nanowires.

PACS numbers: 61.46.+w, 78.55.-m, 81.07.-bKeywords: Nanowires, SiOx, Metal catalyst

I. INTRODUCTION

The discovery of novel materials can usually stimu-late new scientific runners-up and technological break-through. With the development of contemporary sci-ence, one needs to synthesize and to well understandnew materials of reduced dimensions, such as two-dimensional quantum wells, one-dimensional (1D) wires,and quantum dots. Since 1D nanomaterials in the formof tubes, wires, and belts have attracted much atten-tion because of their interesting geometries, novel prop-erties, and potential applications [1–3], considerable ef-forts have been placed on the synthesis and characteriza-tion of those materials over the past several years. Manymaterial systems, including carbon [1,4,5], oxides [6–10],nitrides [11,12], and carbides [13,14] have been success-fully fabricated by using a variety of methods.

Among the successfully fabricated material systems,silicon-oxide (SiOx) nanostructures have attracted con-siderable attention due to their unique properties andpromising application in mesoscopic research, nanode-vices, and opto-electronics devices. Particularly, SiOx

is an important material because of its intense and sta-ble blue light emission at room temperature and, hence,

∗E-mail: hwkim@inha.ac.kr; Fax: +82-32-862-5546

their potential applications in future integrated opti-cal devices [15,16]. Although some researchers demon-strated the production of SiOx nanowires by simply heat-ing the Si wafers to 1300 ◦C [17, 18], the majority ofSiOx nanowires fabrication methods with a relativelylow growth temperature of around 1000 ◦C are catalyst-based methods. In the catalyst-based methods, differentkinds of metal catalysts have been used, such as Au [19–23], Fe [24–27], Ga [28,29], Ni [30], Sn [31], and Co [32].Palladium (Pd) is a platinum-group metal with good cor-rosion resistance and has been studied as a VLSI material[33]. Although carbon nanotubes have been synthesizedusing the Pd catalyst [34–36], to our knowledge, synthe-sis of an inorganic nanomaterial, including SiOx, on aPd substrate has not been reported to date.

In this paper, for the first time we report the pro-duction of SiOx nanowires by a simple heating of Pd-coated Si substrates. We also investigate the structuraland the photoluminescence (PL) properties of the SiOx

nanowires. We discuss a possible growth mechanism withrespect to the role of the predeposited Pd layers.

II. EXPERIMENTS

The preparation processes of the SiOx nanowires wereas follows. We employed Pd-coated Si substrates. In or-

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Fig. 1. (a) Top-view and (b) high-magnification SEM im-ages of the product.

der to fabricate the Pd-coated Si substrates, we used Sias a starting material onto which a layer of Pd with athickness in the range of 3 – 5 nm was deposited by sput-tering. A piece of the substrate was put on top of an alu-mina boat, with the Pd-coated side downwards. Then,the boat was placed at the center of a quartz tube insidethe conventional horizontal furnace. During the experi-ment, a constant pressure with an air flow (∼3.1 % O2 ina balance of argon) was maintained at 300 mTorr. Thesubstrate temperature was set to 1000 ◦C for 2 h. Afterthe synthetic process, the substrate was cooled down andthen removed from the furnace for analysis.

Scanning electron microscopy (SEM) and transmissionelectron microscopy (TEM) images of the product weretaken with a Hitachi S-4200 scanning electron micro-scope and a Philips CM-200 transmission electron micro-scope, respectively. The powder X-ray diffraction (XRD)pattern of the product was obtained with an X’pertMPD-Philips X-ray diffractometer by using CuKα1 radi-ation. Energy-dispersive X-ray (EDX) spectroscopy wasattached to the TEM. TEM samples were prepared bysonicating the substrate in acetone by ultrasonic treat-ment. A drop of the dispersion solution was then placedon a porous carbon film supported on a copper grid. ThePL spectra of the samples were measured in a SPEC-1403photoluminescence spectrometer with a He-Cd laser (324nm, 55 mW) at room temperature.

III. RESULTS AND DISCUSSION

Figure 1(a) shows the SEM top view of the samplemorphology on the Pd-coated Si substrate, indicating

Fig. 2. X-ray diffraction pattern recorded from the prod-uct.

Fig. 3. (a, b) TEM images showing the typical morpholo-gies of SiOx nanowires. (c) Associated SAED pattern. EDXspectra of (d) the nanowire tip and (e) the nanowire stemwith respect to the nanowire shown in Figure 3(a).

that the product consists of bunches of nanowires. Sta-tistical observation of many SEM images indicated thatthe diameters of the nanowires varied from 15 to 100 nm.Figure 1(b) shows a high-magnification SEM image, re-vealing that the cross-sections of the nanowires had acircular shape. Figure 2 presents the XRD patterns ofthe product, showing that the nanowires are fully amor-phous. No reflections are clearly discerned. In the XRDmeasurements, the angle of the incident beam to the sub-strate surface was approximately 0.5◦, and the detectorwas rotated to scan the samples. Therefore, we supposethat the peaks are mainly from the product.

Figures 3(a) and 3(b) show TEM images represent-

Synthesis, Structural Characterization and Photoluminescence· · · – Hyoun Woo Kim et al. -1801-

ing two typical types of nanowires, indicating that bothnanowires are in the form of solid rods. The nanowireshave a straight or slightly curved morphology, agreeingwith the SEM images (Figure 1). The nanowires shownin Figures 3(a) and 3(b) have average diameters of ap-proximately 30 and 18 nm, respectively, with relativelysmooth surfaces. In Figure 3(a), the nanoparticle at thetip of the nanowire appears dark, having high contrastcompared with the nanowire. It is noteworthy that thediameter of the nanoparticle is approximately 7.5 nm,considerably smaller than that of the nanowire stem. AnSiOx layer exists on the surface of the relatively smallnanoparticle at the tip. We surmise that the diameterof the parent droplet was equivalent to the diameter ofthe wire, but the diameter of the ejected droplet was sig-nificantly reduced during the nanowire growth. On thecontrary, the nanowire shown in Figure 3(b) does notcomprise the nanoparticle at its tip.

As Figure 3(c) shows, the highly dispersed selectedarea electron diffraction (SAED) pattern indicates thatthe nanowires are amorphous. EDX measurements madeon the wire tip and the wire stem with regard to thenanowire shown in Figure 3(a) indicate that the nanowiretip consists of Pd, Si, and O (Figure 3(d)) whereas thenanowire stem is composed of only Si and O (Cu and Csignals are generated from the microgrid mesh support-ing the nanowires) (Figure 3(e)). Although we do notknow the exact chemical composition of the nanoparticle,we reveal that the nanoparticle is comprised of elementalPd.

Up to the present, various forms of SiOx nanowireshave been fabricated using the vapor-liquid-solid (VLS)method [23–25, 28, 31, 37] or the vapor-solid (VS) pro-cess [38]. The solidified spherical droplet at the tip of ananowire is commonly considered to be evidence for theoperation of the VLS mechanism, dismissing the growthmechanism mainly controlled by the VS process. Figure3(a) shows the presence of a tip nanoparticle in an as-synthesized nanowire. However, in some instances, thePd-catalyst nanoparticle was not present at the tips ofthe structures (Figure 3(b)), suggesting that the Pd cata-lyst may stay at the bottom of some nanowires during thegrowth process. Similarly, previous work on the produc-tion of carbon nanotubes [39] and MgO nanowires [40]revealed that the growth could be ascribed to the basegrowth mechanism, in which the metal catalyst remainssituated at the bottom of the nanostructures. Hence,the growth mechanism of SiOx nanowires in the presentstudy cannot be ascribed to the commonly-observed tip-growth VLS mechanism only, but must also be ascribedto the base-growth VLS mechanism.

Accordingly, when the Pd-coated substrate was heatedat 1000 ◦C, Pd/Si liquid droplets will form, in which theeutectic temperatures of PdxSi compounds are equiva-lent to or lower than 901 ◦C [41]. The formation of theliquid droplet can be evidenced by the observation thatthe Pd-related nanoparticle has a spherical shape (Figure3(a)). The eutectic compound may begin to form at a

Fig. 4. PL of the SiOx nanowires in which the blue lightemission was revealed to peak at 430 nm and 537 nm.

temperature even lower than the eutectic temperatures,possibly due to the non-equilibrium processing conditionand/or the melting effect of small-sized grains of PdxSiislands, which were agglomerated from the thin (about 3– 5 nm) Pd layer. The liquid-state particles should eas-ily absorb oxygen, and the presence of a relatively smallamount of oxygen is not expected to change the Pd-Siphase diagram significantly. As the droplets or particlesbecome supersaturated, SiOx nanowires are formed, pos-sibly by the reaction between Si and O. The droplet willcontinuously absorb Si atoms as it is abundant in the Sisubstrate. The most likely source of oxygen may comefrom the O2 in the carrier gas while the oxygen adsorbedon the Si wafer is due to air exposure during the process-ing and the residual oxygen in the tube can be due toother sources.

Figure 4 shows the PL spectrum of the SiOx nanowiresmeasured at room temperature, which is an apparentbroad emission band mainly located in the visible region.A Gaussian fitting analysis showed that the broad emis-sion band was a superimposition of two major peaks, oneat 430 nm and the other at 537 nm. A similar blue emis-sion with a peak position in the range of 414 – 470 nmhas been previously observed in the PL spectrum of SiOx

nanowires [21,23,24,42], which was ascribed to neutraloxygen vacancies or oxygen deficiency-related diamag-netic defect centers [24]. Also, a similar green emissionpeaking at around 540 nm has been previously reportedfrom SiO2 nanowires [17], which was attributed to neu-tral oxygen vacancies [43]. We, thus, believe that the PLemission from the SiOx nanowires in the present studycan be attributed to the above-mentioned defects arisingfrom an oxygen deficiency, presumably being generatedduring the high-temperature synthetic process.

IV. CONCLUSION

In summary, amorphous SiOx nanowires have beenfabricated via a Pd-catalyzed process. We have ap-

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plied XRD, SEM and TEM techniques to characterizethe structures of the samples. The as-synthesized SiOx

nanowires have an amorphous structure with average di-ameters in the range of 15 – 100 nm. The growth ofSiOx nanowires is most likely controlled by the extendedVLS mechanism, in which Pd-related catalytic particlesare attached to the tips of some nanostructures. ThePL measurement with a Gaussian fitting shows appar-ent blue light emission bands peaking at 430 and 537nm.

ACKNOWLEDGMENTS

This work was supported by an Inha University re-search grant.

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