Controlled Growth of Silicon Oxide Nanowires from a Patterned Reagent

Feng Wang,*,†,‡ Marek Malac,†,‡ Ray F. Egerton,†,‡ Alkiviathes Meldrum, † Peng Li,†Mark R. Freeman,†,‡ and Jonathan G. C. Veinot§

Department of Physics, UniVersity of Alberta, Edmonton T6G 2G7, Canada, National Institute forNanotechnology, 11421 Saskatchewan DriVe, Edmonton T6G 2M9, Canada, and Department of Chemistry,11227 Saskatchewan DriVe, UniVersity of Alberta, Edmonton T6G 2G2, Canada

ReceiVed: NoVember 14, 2006; In Final Form: December 18, 2006

A new process to fabricate silicon oxide nanowires (NWs) from a patterned reagent is reported. Arrays ofNWs grow on patterned nanodots containing exposed hydrogen silsesquioxane (HSQ)/Fe-SiO2 nanocompositesduring annealing at 900°C. The NWs were seeded by metallic iron nanoparticles, and the resultingmicrostructure and morphology of the NWs is directly related to the size of the individual iron nanoparticles.The growth process could be dominated by a solid-state transformation mechanism in which iron nanoparticles,originally embedded in a SiO2 matrix, diffuse to the surface and act as nucleation sites, the exposed HSQbeing the source for the growing NWs.

Silicon-based nanowires, including crystalline and amorphoussilicon and silicon oxide nanowires, have shown promisingapplications in nanoelectronics and integrated optic devices, suchas low dimensional waveguides for functional microphotonics,scanning near field optical microscopy, optical interconnectson optical microchips, biosensors, and optical transmissionantennae.1-3 Synthetic methods including chemical vapordeposition,4 laser ablation,3 sol-gel,5 and thermal evaporation,6

have been used for random (i.e., nonpatterned) growth ofnanowires (NWs). However, for many potential applications,it is desirable to control the position and size of the NWs sothat post-growth manipulation is not required.7,8 Recently,selective growth of silica NWs via a vapor-liquid-solidmechanism9 was achieved using an ion implantation mask.10

Here we show that patterning with much higher (by severalorders of magnitude) spatial resolution can be achieved byemploying electron-beam lithography. The diameter of the NWsis determined by the iron nanoparticles encapsulated at the endof individual NWs, similar to other reports in Si and GaPnanowires,11,12 but in our case, the NWs grow directly from apatterned reagent, and inflow of a Si-containing gas is notrequired.

The fabrication procedure can be divided into three stages:(i) formation of the Fe-SiO2 nanocomposites (i.e., iron nano-particles embedded in a SiO2 matrix), (ii) patterning of thereagent, and (iii) the growth of the silicon oxide nanowires. Fe-SiO2 nanocomposites were fabricated by electron-beam deposi-tion of SiO2 (purity ) 99.999%) and Fe (99.95%) to form aSiO2/Fe/SiO2 trilayer on 50 nm-thick silicon nitride (Si3N4)membranes and annealed in situ at 880°C, in an ultrahighvacuum (UHV) system with base pressure 10-10 Torr.13 A 6nm-thick carbon film was pre-coated on the Si3N4 membrane

to suppress charging during electron beam lithography. Fol-lowing fabrication of the Fe/SiO2 composite, a 60 nm-thick layerof hydrogen silsesquioxane (HSQ; Fox17, Dow Corning, whichis a well-known negative planarizing electron beam resist) wasspin-coated onto the sample surface. Regular arrays of nanodotsof desired dimensions were patterned in the HSQ layer using aRaith 150 electron-beam lithography system. Areas of the Fe/SiO2 nanocomposite not protected by the exposed HSQ weresubsequently removed by dry etching in an argon ion mill,leaving the Fe/SiO2 nanocomposite film only in areas underthe exposed HSQ dots. The samples were then annealed for 1h at 900°C in a slight overpressure of N2/H2 (95%/5%) forminggas. Microstructural and chemical investigations were carriedout in an analytical transmission electron microscope (TEM,JEOL 2010), using bright-field imaging, electron energy-lossspectroscopy (EELS) and energy-loss near-edge structures(ELNES).

An array of circular nanodots of 50 nm diameter and 1µmspacing, is shown in Figure 1a. A limited number of ironnanoparticles (typically less than 5) are present within eachnanodot (see, in the inset of Figure 1a); Figure 1b shows a TEMimage of an array of NWs grown from the patterned nanodots.NWs grew only from the original patterned HSQ/Fe nanodots,leaving the areas between the dots bare and completely free ofNWs. The inset of Figure 1b shows NWs produced from atypical dot, with higher magnification.

The length of the NWs on the nanodots is less than 1µm inthis experiment. It is determined by the supply of the stockmaterial and this is controllable, so the length of the NWs canbe further tailored in accordance with the requirements ofpractical applications. It is worth noting that the number of ironnanoparticles is determined by the size of each nanodot andthe areal density of Fe nanoparticles. Therefore, it is expectedthat a suitable choice of Fe nanoparticle growth parameters andHSQ dot diameters can be used to control the number of iron

* Corresponding author. E-mail: fwang@phys.ualberta.ca.† Department of Physics, University of Alberta.‡ National Institute for Nanotechnology.§ Department of Chemistry, University of Alberta.

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nanoparticles and consequently the number of NWs grown fromeach dot. Although the average number of NWs on each nanodotcan be controlled, the possibility to grow one individualnanowire from each dot was not demonstrated to date.

Using a thick Si3N4 membrane as substrate poses difficultiesfor TEM investigation; however, this problem was avoided byexamining NWs that were suspended between gaps in a brokenSi3N4 film, allowing microstructural and chemical analysiswithout influence from the substrate. Figure 2 shows a TEMimage of a bundle of NWs protruding into vacuum from theSi3N4 substrate. The nanowires have a smooth surface and aclosed end containing a single iron particle. The nanowires havediameters of 10-30 nm, comparable to the diameter of the iron

nanoparticles encapsulated at the nanowire tips. TEM imagingand diffuse rings in electron-diffraction pattern (not shown here)collected from the body of nanowires showed that the nanowiresare noncrystalline. Indexing the electron diffraction patterncollected from the particles at the end of the NWs (inset inFigure 2) indicated that bcc-Fe is the main phase; no reflectionsassociated with iron oxides were found.

The diameters of individual nanowires and encapsulatednanoparticles are correlated. A total of 98 nanowires weremeasured from plan-view TEM images for a quantitativecomparison. Figure 3 shows the relation between nanowirediameter and the iron particle diameter. The solid line is a linearleast-square fit. The results suggest that the diameter of thenanowires is predominantly determined by the iron nano-particles, which leads to the possibility of synthesizing diameter-controlled silicon oxide nanowires. Control of Fe seed particlesize has been demonstrated through a multiple-layer synthesisprocedure.13

Analysis of electron energy-loss and Auger spectra showedthat the nanowires are composed of silicon and oxygen. Tofurther clarify the chemical environment of the silicon in thenanowires, the silicon L23 and oxygen K-ionization edges weremeasured from individual nanowires protruding from thesubstrate (shown in Figure 2). The Si L23 edge shows a differentfine structure than that recorded from nanowires on the Si3N4

substrate (where the spectra are dominated by the substrate),and no nitrogen K-edge was discerned in the energy loss spectra.The integral intensities of oxygen K and silicon L-edges alloweddetermination of the [O]:[Si] ratio, using O-K and Si-Lionization cross-sections calculated from SIGMAK3 and SIG-MAL3 programs,14,15and the resulting atomic [O]:[Si] ratio was1.7( 0.1. In addition, the near-edge fine structure of the siliconL23-edge of SiO2 and elemental Si differ near the thresholdenergy,16,17 negating the possibility that the wires containelemental Si.

The morphology of the NWs (with metal particles attachedto the end) and the linear relation between the diameters of thenanowires and the encapsulated nanoparticles suggest theoperation of a classical vapor-liquid-solid (VLS) growthmechanism.9 Nevertheless no vapor source such as silane (SiH4)or other gaseous silicon compound was provided in the presentgrowth process. Furthermore, the presence of liquid irondroplets, on which gaseous silicon precursor might producesilicon for the growth of nanowires,2,9,11is unlikely in this systembecause the annealing temperature used (900°C) was muchlower than both the melting point of iron and the eutectic pointfor iron-silicon or iron-silicon oxides,18 even when the surface

Figure 1. Bright-field TEM images for (a) arrays of iron nanoparticlesafter lithographic patterning of 50 nm diameter and 1 um spacing (theinset shows one nanodot containing several nanoparticles) and (b) arraysof nanowires grown from the patterned nanodots (the inset showsnanowires on a nanodot).

Figure 2. Bright-field TEM image of a bundle of individual nanowiresprojecting from the substrate, with iron nanoparticles encapsulated(darker region) at the NW tips. The insets are selected-area diffractionpatterns of iron nanoparticles that are consistent with the bcc-Fe phase.

Figure 3. Dependence of the diameter of the nanowires on that of theencapsulated iron particles. The solid line is a fit to the measured data:y ) 0.96x + 2.48 (y ) diameter of the nanowire;x ) diameter of thenanoparticle (exclusive of the shell); the dotted liney ) x is used forcomparison.

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energy of the cluster is considered.19,20Therefore, it is unlikelythat the present nanowires grew by a VLS-like mechanism. TheSLS (solid-liquid-solid) mechanism can also be excluded, inview of the top-growth mode and the low processing temperaturein this growth process.21

The direct solid-state transformation mechanism has beenproposed for the formation of amorphous silicon oxide nano-wires from silica films, in which nickel nanoparticle nucleatingcenters are formed on the surface from nickel atom diffusionthrough a reduction layer.22 This mechanism provides a possibleexplanation for the growth observed in our experiments. Ironnanoparticles, initially embedded in the SiO2 matrix, diffuse tothe surface when the sample is heated to 900°C. The formationof the iron nanoparticle on the surface has been observed inour previous experiments;23 the diffusion mechanism remainsto be determined. Fe particles at the exposed HSQ surface actas NW nucleation sites. NWs nucleate underneath the ironnanoparticles and grow by the diffusion of the Si atoms fromthe underlying exposed HSQ.

Our proposed mechanism is summarized in Figure 4. HSQis a well-defined cage molecule with an empirical formula(HSiO3/2)n. It has been reported, in the absence of conclusivestructural characterization, that, upon exposure to electron-beamirradiation, some of the Si-H bonds are broken and an insolublecrosslinked siloxane network is formed.24 This ill-defined,crosslinked network is expected to collapse upon annealing at900 °C to form a “SiOx-like” film containing regions rich inlow-valent Si. The diffusion and nucleation of oxide-encapsu-lated, low-valent Si on metal nanoparticles has been reported.25

In this regard, it is reasonable that the present “SiOx-like” film,produced from annealing exposed HSQ, enhances the nucleationand one-dimensional growth of the silicon NWs, in a similarfashion to that proposed for “SiO” by Wang et al.25 In thepresent system, as-grown nanowires likely form as noncrystal-line Si nanowires that subsequently oxidize rapidly to SiO2 uponexposure to the ambient atmosphere.

In support of this proposed mechanism, control experimentsusing an Fe/SiO2 nanocomposite, with no overlaying exposedHSQ layer as well as with an Fe/SiO2 nanocomposite bearingan unexposed HSQ film, afforded no NWs when annealed underidentical conditions. These experiments clearly show that acrosslinked SiOx-layer obtained from electron-beam exposureof HSQ is crucial to the formation of the present NWs. Atpresent, the exact role of the crosslinked HSQ remains unclear.Structural analysis of beam-crosslinked HSQ films as well asan in situ study of as-grown nanowire structure and chemicalcomposition are expected to provide valuable information

regarding the nanowire growth mechanism and are subject offurther investigation.

In summary, a process for fabricating arrays of silicon oxidenanowires from a patterned reagent has been developed, inwhich the diameter of the nanowires depends on the nano-particles attached at the end. Such an unusual growth process,being unexplainable by the VLS and SLS mechanisms, may beattributed to direct solid-state transformation.

Acknowledgment. This work was supported by NRC andNSERC. The electron-beam lithographic patterning was doneat Nanofab in the University of Alberta. The authors thank Dr.Ken Bosnick and Dr. Yucheng Lan for helpful discussions andDon Mullin and Greg Popowich for technical assistance. F.W.acknowledges support from the Killam Trust.

References and Notes

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Figure 4. Proposed growth process of nanowires: (a) as-producedFe-SiO2 nanocomposites, with spin-coated HSQ on top; (b) migrationof iron nanoparticles onto the top of the nanodots; (c) diffusion of Siatoms; (d) growth of nanowires onto the surface of nanodot.

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