Fast vapor phase growth of SiO 2 nanowires via surface-flow on Ag core / SiO 2 shell structure

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I. INTRODUCTION
3][4][5] It involves various intervening processes which differ from case to case, thus it allows a versatile design of the routes for the growth of nanowires of different compositions and geometry. 3,6 he basic idea of VLS mechanism is that precursor atoms or molecules from vapor phase preferentially adsorb on and dissolve into the metal droplet under eutectic condition and then transport across the droplet to eventually precipitate to form the anticipated solid nanostructure, in which various thermodynamical and physiochemical factors may play a decisive role in determining the final product of the method. 7,8 bviously, the capture of precursors, for molecular precursors also their dissociation, the transport of growth materials and precipitation are the dominant rate-limiting factors, while the way of supply and the status of the catalyst as well as its interaction with the surrounding materials determine the features of resulting wires. 6,9 n those diffusion-dominated VLS models, precursors from the vapor phase first land on the surface of the catalyst droplet and then diffuse via hopping of individual atoms across the surface or through the bulk. 10,11 hus, the nanowire growth rate hangs on the concentration gradient and diffusion coefficient of the precursor elements.Surface diffusion generally dominates the growth process in the cases of small droplet, poor solubility of growth species in catalyst, and large ratio of surface-to bulk diffusion coefficient. 10Mass transport via surface can lead to a larger growth rate, thus favorable for the fast production of nanowires.
The growth conditions for VLS mechanism exclude in-situ monitoring for most research aims, therefore the mechanism based on the observation of end products often contains some speculated details.Although there was report on in-situ transmission electron microscope monitoring of nanowire growth via VLS mechanism, which revealed as a surprise that Ge nanowires can grow a To whom correspondence should be addressed.E-mail: zxcao@iphy.ac.cn 2158-3226/2012/2(1)/012187/7 C Author(s) 2012 2, 012187-1 below the eutectic temperature with either liquid or solid Au catalyst, 6 but the studies under transmission electron microscope are limited to Au-catalyzed growth for which the low temperature and low pressure condition is feasible.Other situations concerning nanowire growth at T < T E may involve a quasisolid or quasiliquid particle on the tip of the nanowire, thus termed VQS mechanism, where the growth rate cannot be explained by material transport on a solid particle. 12In the opposite direction, there must be other stories in case the growth proceeds under high temperature and/or high pressure conditions.Any method that allows accessing the intervening processes in a particular circumstance helps improve the understanding of this mechanism.Furthermore, the catalyst particles involved in VLS growth are often actualized via agglomeration from a pre-deposited film, 10,13 which requires a good wettability of the catalyst metal to substrate.The assembly of catalyst particles from a pre-deposited film also intermingles with the rooting of the nanowires, causing some additional difficulties to the implementation of the VLS mechanism. 13A controllable supply of catalytic particles, if also the rooting ability of the particles can be thus enhanced, is clearly advantageous.
In this article we report the fast growth of uniform, amorphous, millimeter-long SiO 2 nanowires via viscous flow of SiO 2 layer on a preformed Ag core/SiO 2 shell structure, where both the precursory SiO 2 molecules and the catalyzing Ag particles come from co-evaporation of Ag 2 O and SiO powders.This method, among other advantages, allows a flexible control of the nanowire diameter.Furthermore, the 'frozen' scenario on the sample obtained by cooling the system at different growth stages, thus aborting the growth process, allows detailed mechanism investigation.

II. EXPERIMENTAL
Commercial powders of SiO (99.7%) and Ag 2 O (99.99%) were mixed in a mass proportion of 1:2 as precursor.The powder mixture was put in a columniform ceramics crucible with an inner diameter of 1.5 cm and a depth of 2.5 cm.A clean Si slab substrate, 3.0 cm x 0.5 cm in size, was inserted aslant in the crucible to collect the evaporation product-a mild temperature gradient is expected to develop along the substrate so as to enhance the opportunity of success of the experiment.Evaporation was performed with the crucible mounted in a closed reaction chamber which was preevacuated to a pressure below 0.1 torr and then filled with the gas mixture of Ar (90%) and H 2 (10%) to a pressure around 300 torr.The base pressure determines the size of the Ag core/SiO 2 shells formed in the vapor, thus can be intentionally adjusted to control the diameter of the resulting SiO 2 nanowires.A graphite plate placed under the crucible was used as the heating element.For the evaporation experiment here discussed, the crucible was heated to 1100 • C∼1200 • C (monitored at the bottom of the crucible) at a heating rate of about 50 • C/min.Accordingly, the temperature measured atop the backside of the Si substrate fell in the range of 840 • C∼950 • C (see Fig. S1 in supplemental material 25 ), under which the fluidity of SiO 2 on Ag droplet suffices to induce a flow towards the substrate to form nanowires.After the evaporation had lasted for about 10 minutes or so at the preset temperature, power supply was cut off to let the system cooled down to the room temperature by itself.The cooling rate at the beginning was up to 100 • C/min.As the precursor Ag 2 O powder will be exhausted earlier due to the preferential evaporation, noting that Ag 2 O deoixation, begins already at ∼300 • C, so only SiO 2 molecules are available for the growth of nanowires in the later stage of evaporation.][16][17] The deposits were ex situ investigated by using a transmission electron microscope (Tecnai F20) and a field-emission scanning electron microscope (FE-SEM, XL30) which also enables energydispersive X-ray spectroscopic analysis of the samples.The SEM images record the evaporation products that were frozen at a chosen moment, therefore those products on one substrate but having different history of life would reveal the story of Ag catalyzed growth of SiO 2 nanowires unfolding at different phases.By combining the SEM images taken on different positions of a substrate, a motion picture could be made for the growth process.To emphasize, the scenario we see under the SEM is the scene frozen at a particular moment that all the components of the growing structures are found in solid state, whereas in the actual situation some of them are melt.

III. RESULTS AND DISCUSSION
By using the method described above, millimeter-long SiO 2 nanowires of variable diameter can be grown at a very large growth rate, with a wire density sensitive to the substrate temperature (see Figs.S2-S3 in supplemental material 25 ). Figure 1 displays the scanning electron microscope (SEM) image of a sample prepared at 1100 • C for 18 minutes and without filling the pre-evacuated chamber with Ar+H 2 -a low chamber pressure helps suppress the diameter of the nanowires (see discussion below).Even assuming that the nanowire growth begins immediately with the heating of evaporants, the growth rate is still much larger than 10 nm/s.The diameter of the nanowires in Fig. 1 can be as small as 60 nm.The nanowires are quite uniform throughout its length up to millimeters, and owing to such a length the constantly vibrating nanowires assume a diffused profile under SEM.The nanowires terminate with a metal-rich head, indicating that the nanowires might have grown via the VLS mechanism.By breaking off the growth process earlier, the initial stages of the growth can be revealed.
Under given conditions, a dense vapor of Ag was generated via decomposition of Ag 2 O, which could be assisted in the presence of SiO and H 2 .Meanwhile, the vapor contains also a lot of SiO 2 molecules which partially occurs through the deoxidation reaction 2SiO → Si + SiO 2 and direct oxidization by residual oxygen in the chamber.Noting that in a gas at 1100 • C∼1200 • C and at a pressure of ∼300 torr, the mean free path for the molecules is less than 1μm.Enormous collision events take place on the path from the evaporating surface to the substrate.Ag atoms and SiO 2 molecules collide and stick together to form large particles which keeps growing through absorption of more atoms and molecules and/or through coalescence with other particles.5][16][17] That the core is Ag instead of the AgSi eutectic alloy can be verified from TEM, X-ray diffraction and energy-dispersive X-ray analysis results. 19,20 he size and shell thickness for such core/shell structures can be adjusted by changing the composition of the precursor and the evaporation conditions for the study of many interesting phenomena including the formation of stressed Fibonacci parastichous spirals. 14,16  order to resolve the detailed mechanism for the Ag-catalyzed growth of SiO 2 nanowires, in the following we focus on the results obtained with the evaporation chamber pre-filled to 300 torr, thus the Ag droplets formed in vapor could be one micron in size (much larger than in the case shown in Fig. 1).Fig. 2 displays some Ag core/SiO 2 shell structures with a relatively thinner shell.Such a core/shell structure, when experienced fast cooling, would show craters and crumples in the shell due to the serious shrinkage of the liquid Ag core (Fig. 2(a)).When an Ag core/SiO 2 shell comes into contact with the Si substrate, the draining effect arising from good wetting pierces the SiO 2 shell from the antipode of the contact point.The two core/shell structures presented in Fig. 2 (cf.Fig. 3(d))-the shell has receded considerably towards the substrate that the Ag core was half exposed.The SiO 2 shell at the moment of peeling was evidently in the fluidic state, since the receding front shows clearly a notched rim-a typical result of fingering instability for liquids.This confirms that before being frozen to be imaged, the SiO 2 shell was undergoing fluid motion.
As is well known, the amorphous SiO 2 begins to exhibit viscous flow at ∼950 • C 21 and the dominant transporting process for SiO 2 glasses is viscous flow even at nanometer scale. 22,23 hen the core/shell structure just falls on the Si substrate, the outer SiO 2 layer wets the substrate and forms a concave neck around the contact point.Compared to the positive curvature of SiO 2 layer covering the Ag core, the negative curvature in this neck region implies a smaller chemical potential, according to the Kelvin equation.It can cause mass transport from the core/shell surface towards the contact region, and eventually peel off the liquid SiO 2 layer wrapping the Ag core which flows to the neck region to initiate nanowire growth.The thickness of the resulting nanowire is therefore determined by the size of the Ag core which can be easily controlled by adjusting the chamber pressure.
Figs.3(a)-3(c) display some core/shell structures which have a little longer history than those presented in Fig. 2-the SiO 2 shells now have been peeled off completely, and the Ag droplets are totally exposed to the vapor.The polyhedral morphology of the Ag particles showing {111}facets with spiral steps is the result of cooling (Fig. 3(a))-this finding can be applicable for the facet-controlled growth of Ag microcrystals and has been discussed elsewhere. 20Due to the small solubility of SiO 2 in Ag, the Ag particle and the SiO 2 'pedestal' have only a weak bond, some Ag particles could have been removed by the stress arising from cooling, leaving behind an empty pedestal (see Fig. S4 in supplemental material 25 ) which can be used to estimate the original shell thickness.For the core/shell structures giving rise to Fig. 3(a), the original shells are ∼50 nm thick, i.e., about hundred times the molecular size.In this circumstance, the liquid SiO 2 film can be treated as a continuous medium.
More SiO 2 molecules adsorbed onto the Ag droplet sustain the liquid SiO 2 layer which keeps flowing to the contact zone.This will elongate the SiO 2 part between the substrate and the upraised Ag particle-a key step for fast VLS growth of nanowires with the current system.Thus the SEM image for the sample at this stage shows a scenario full of Ag particles sitting on elongated SiO 2 supports, as golf balls sitting on the tee.Remarkably, the SiO 2 neck beneath the Ag particle in Fig. 3(b) is a circular column whereas in Fig. 3(c) it is in the shape of left-handed spiral. 3,4 his spiral arises from the typical spiraling of fluid as observed in bath water around the plughole, which confirms the excellent fluidity of SiO 2 film on Ag droplet under given temperature.The spiraling wires in Fig. 3(c) occur only when abundant supply of SiO 2 is available, as confirmed by the floccules on the Ag particle arising from the continuing SiO 2 adsorption during the cooling stage.This tells us that in order to grow a rectilinear nanowire via surface flow, the fluidity of the growth materials on the catalyst should be well controlled.In fact, a steady-state viscous flow for SiO 2 should be maintained, as balanced by adsorption on the front of catalyst and precipitation on the liquid-solid interface.
The aforementioned facts point towards surface viscous flow as the mass transport path responsible for the fast growth of SiO 2 nanowires here.Fig. 3(d) exhibits the TEM image of the head of a typical wire, showing a spherical Ag core, the Ag-SiO 2 interface and the peripheral (once) fluidic SiO 2 layer, a (once) liquid neck zone of positive curvature attaching to the relatively thinner solid wire in a configuration corresponding to the S8 structure in Ref. 3. Due to the fact of fast SiO 2 deposition, the resulting nanowire is obviously amorphous.Important parameters for this nanowire head are the diameter of Ag droplet (∼ 270 nm), the diameter of SiO 2 nanowire (∼ 170 nm), the thickness of peripheral SiO 2 layer (∼10 nm), plus the thickness in axial direction of the convex neck zone (∼80 nm) and its maximum lateral dimension (∼250 nm) (thus here the SiO 2 has still a smaller chemical potential than on the spherical catalyst).If nanowire grows via bulk diffusion, assuming a conservative growth rate of 1 nm/s and a concentration gradient of 0.01%/nm in the Ag droplet, which makes a difference of 2.7% in total, it demands an unrealistic bulk diffusion coefficient of 4 × 10 −11 cm 2 /s.Surface diffusion cannot achieve so high a growth rate either.Even to the viscous flow mechanism, this requires a flow rate of ∼11.0 nm/s as estimated at the equator of the Ag droplet.The fast viscous flow on the catalyst droplet may lead to a spinning growth of nanowires, a phenomenon that occurs quite often 3,4 but has not received the deserved attention.The sinewy Si/Ag alloy interface helps suppress the convection of Ag droplet by the SiO 2 flow, protecting the catalyst from being consumed under most growth situations.No crystalline Si could be identified in the wires.
Thus the detailed VLS mechanism for the Ag-catalyzed growth of SiO 2 nanowires in the current work becomes clear, as illustrated in Fig. 4. Initially, Ag core/SiO 2 shell structures in liquid status are formed in vapor (Fig. 4(a)).Upon arriving at the substrate, the liquid SiO 2 shell is pierced to expose the Ag droplet to capture more SiO 2 molecules; [14][15][16] the fluidic SiO 2 layer is driven by the draining effect to form a columnar support for the Ag droplet (Fig. 4(b)), which accomplishes the rooting of nanowire in substrate.In the subsequent growth stage (Fig. 4(c)), more SiO 2 molecules dissolve into the peripheral liquid SiO 2 layer to maintain a steady viscous flow to the neck zone where they become supersaturated and precipitate, resulting in an elongated nanowire.
The validity of surface viscous flow as mass transport path here could also be confirmed by the following fact.In the Si/SiO 2 nanowires, 24 periodic chains of Au nanoparticles were observed, obviously the fast flow of SiO 2 layer encasing the Au/Si alloy droplet there deformed and tore the metal droplet.This phenomenon can also be observed in the current system when the growth was performed under 1200 • C that an overly abundant supply of SiO 2 was available, some nanowires would be found containing a chain of Ag particles (see Fig. S5 in supplemental material 25 ).Clearly, in such a circumstance nanowire growth may be terminated suddenly.
The critical point for the fast growth of ultra-long uniform nanowires with the current method is the substrate temperature.For a successful rooting and a fast growth via viscous flow, a high temperature is demanded to achieve good fluidity for the precursor materials.On the other hand, the substrate temperature should be sufficiently low so that the growth materials from the metal droplet can turn into a solid wire.This explains why in the current experiment there were always nanowires on the hotter end of the Si substrate, whereas on the cooler far end of the substrate, nanowires were seldom (see Fig. S3 in supplemental material 25 ).
The current VLS method has a few other advantages.First it does not need a catalyst-coated substrate, thus nanowires can be grown onto a substrate that catalyst does not wet.Since the nanowire growth begins with the wetting of the substrate with the growth material in liquid status, consequently the patterned growth of nanowire can be realized by structuring the substrate simply according to its wettability to the growth material.Furthermore, catalyst overgrowth can be easily avoided since it is provided from a distant evaporating surface.This is very important for the attainment of uniform nanowires.Another interesting phenomenon to remark is that since the nanowire growth begins with a liquid Ag core/SiO 2 shell coming into contact with substrate, it is then of no surprise that when the core/shell touches an uneven substrate at two points, one Ag particle may simultaneously catalyze two wires (see Fig. S6 in supplemental material 25 ).

IV. CONCLUSION
In summary we demonstrated a process with which uniform, millimeter-long SiO 2 nanowires can be grown at a high growth rate by co-evaporating Ag 2 O and SiO powders.The wetting of the substrate by liquid Ag core/SiO 2 shell structure, which frees the Ag droplet to act as catalyst, initiates the subsequent growth of amorphous SiO 2 nanowire.Mass transport via a viscous flow boosts the growth rate.This method is quite advantageous concerning the easy control of nanowire diameter, enhanced rooting ability, suppression of catalyst overgrowth, etc.It can be suggestive for the implementation of VLS mechanism to grow nanowires from other material systems.

FIG. 1 .
FIG. 1. SEM micrograph showing millimeter-long SiO 2 nanowires grown from co-evaporation of SiO and Ag 2 O, with the Ag droplet previously formed in the dense vapor acting as catalyst.The wires in the inset have a width of ∼60 nm.Evaporation temperature: 1100 • C; duration: 18 minutes.
FIG. 2. SEM micrographs showing the Ag core/SiO 2 shell structures just arriving at the substrate.(a) Two unbroken core/shell structures.The shells become wrinkled due to the mismatched contraction of the core and the shell; (b) Two core/shell structures with the SiO 2 shell wrapping the Ag core largely peeled off due to the draining effect.The receding front of the broken SiO 2 shells shows clearly a notched rim resulting from the fingering instability, a typical behavior for liquid.

FIG. 4 .
FIG. 4. Schematic illustration of VLS mechanism for the growth of SiO 2 nanowires by co-evaporating Ag 2 O and SiO.(a) An Ag core/SiO 2 shell structure in liquid status flying towards the cooler substrate; (b) The fluidic SiO 2 film encasing the Ag droplet pierced at the apex due to the draining effect from its contact to the substrate, forming a columnar support for the now exposed Ag droplet; (c) Subsequent incorporation of more SiO 2 molecules into the Ag droplet maintaining the viscous flow to elongate the SiO 2 nanowire.The dashed line indicates the solid-liquid interface of SiO 2 ; (d) At the end of growth when no more SiO 2 is supplied, a rather clean Ag particle is left at the head of the nanowire.The various aspects in the figures are not to scale.