Hot-Filament-Assisted Growth of Straight SiOx Nanowires for

Uniform and straight amorphous SiOx nanowires with a length of several micrometers and an average diameter of 100 nm were synthesized by directly heat...
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Hot-Filament-Assisted Growth of Straight SiOx Nanowires for Optoelectronic Application Tian-Xiao Nie,†,‡ Zhi-Gang Chen,‡ Mu-Tong Niu,∥ Jonathon Wu,‡,§ Jin-Ping Zhang,∥ Yue-Qin Wu,‡ Yong-Liang Fan,† Xin-Ju Yang,† Zui-Min Jiang,*,† and Jin Zou*,‡,§ †

State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China Materials Engineering and §Centre for Microscopy and Microanalysis, The University of Queensland, QLD 4072, Australia ∥ Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215125, China ‡

ABSTRACT: Uniform and straight amorphous SiOx nanowires with a length of several micrometers and an average diameter of 100 nm were synthesized by directly heating GeSi alloy film substrate with high melting-point tungsten. Systematically comparative experiments suggest that both the tungsten and GeSi alloy film play an important role in the formation of straight amorphous SiOx nanowire. Through detailed morphological, structural and chemical characterizations using electron microscopy, the contact angle anisotropy mechanism is suggested for the growth of the straight SiOx nanowires.

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flower-like,25 carrot-like,26 comet-like,26 and fishbone-like14 morphologies have been successfully fabricated using micrometer-sized Ga or Sn particles as catalysts, which exhibit a feature that one catalyst simultaneously guides enormous SiOx nanowire growth. However, for nanodevice applications, precisely controlling the size and direction of individual nanowires is essential. This is particularly critical for the amorphous nanowires because the one-to-one relationship between a catalyst and its underlying nanowire has not been observed so far. In this study, we demonstrate a new approach to synthesize straight SiOx nanowires with a bulk quantity by using a highmelting-point tungsten filament with the experimental design illustrated in Figure 1a. One advantage of this approach is that by comparing the tungsten contacting area and the area far away from the tungsten on a single substrate the impact of tungsten on the nanowire growth can be clarified. Through carefully designed comparative experiments, the possible growth model corresponding to this extraordinary finding is proposed.

INTRODUCTION One-dimensional nanomaterials, such as nanotubes, nanowires, and nanobelts, have attracted enormous attention because of their interesting geometries, novel physical properties and potential applications in nanoscale electronics and optoelectronics.1−5 Considerable efforts have been devoted to the synthesis and characterization of nanomaterials in the past decade. Up to now, nanowires with different elements and compounds, such as Si,2 SiOx,6 C7 and ZnO,8 have been successfully fabricated by a variety of methods. Among them, SiOx nanowires have attracted great attention in recent years because of their intense and stable blue-light emission at room temperature9,10 and hence their potential applications in future integrated optical devices, such as in the field of localizations of lights,11 low dimensional waveguide,12 and scanning near-field optical microscopy.12,13 Different methods have been used to synthesize SiOx nanowires including chemical vapor deposition,14,15 laser ablation,9 and solid-state reaction in the presence of different metal catalysts.16−18 All of these growth methods fall into the underlying growth mechanisms: vapor−liquid− solid (VLS),19,20 solid−liquid−solid (SLS),16 and oxide assistant growth.21 In oxide assistant growth mechanism, the silicon suboxide clusters aid the growth of amorphous SiOx nanowires. In the SLS mechanism, the synthesized SiOx nanowires were dense and tangled together,16,18,22 and their metal catalysts, such as Au,22 Ni,16 Fe,23 and Pt,18 remained on the surface of the Si substrate during the entire growth process as the roots for SiOx nanowires growth. Until now, most of the reported SiOx nanowires were ascribed to this growth mechanism. In the VLS mechanism, one representative growth feature is that a catalyst keeps on the tip of the nanowires to guide and control their growth.24 So far, SiOx assemblies with © XXXX American Chemical Society

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EXPERIMENTAL SECTION A GeSi alloy thin film of ∼30 nm thickness was grown on a ptype Si (100) substrate with a resistivity of 5−10 Ω cm in a solid source molecular beam epitaxy (MBE) system (Riber Eva32) with two electron-beam evaporators for Ge and Si sources, respectively. After they were cleaned by using the Shiraki method,27 the Si substrates were loaded into the MBE growth Received: April 11, 2013 Revised: May 20, 2013

A

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Figure 1. (a) Schematic illustration describing the position of tungsten filament on the substrate. (b−d) SEM images of synthesized straight nanowires collected from the position A indexed in Figure 1a from low resolution to high resolution. (e,f) SEM images of the synthesized curved nanowires collected from the position B indexed in panel a in the area far-away from the tungsten filament.

chamber with a base pressure better than 5 × 10−10 Torr. After heating the substrate to 1000 °C for 10 min, a clean Si surface with (2 × 1) reconstruction pattern, confirmed by the reflection high-energy electron diffraction, was obtained. Subsequently, the substrate temperature was lowered to 650 °C, and a 20 nm thick Si buffer layer was grown at a growth rate of 0.08 nm s−1. After the substrate temperature was further lowered to 550 °C, a nominal 30 nm thick Ge0.3Si0.7 alloy thin film was deposited on the Si substrate. The synthesis of the SiOx nanowires was carried out in an annealing furnace equipped with a temperature controller and a quartz tube. 1100 °C was chosen to grow SiOx nanowires. The annealing furnace was heated to 1100 °C in the presence of forming gas (95% N2 + 5% H2) for eliminating the residual impurity gases in the quartz tube. Subsequently, the GeSi alloy thin film sample covered with one tungsten filament (as illustrated in Figure 1a) was inserted into the middle of the tube and kept for 30 min in forming gas with the flow rate of 150 L h−1, then followed by cooling to room temperature naturally in the tube. The morphologies of the synthesized products were analyzed by scanning electron microscopy (SEM, JEOL JSM-6400F). Transmission electron microscopy (TEM, FEI Tecani F20 equipped with energy-dispersive spectroscopy (EDS) and electron energy-loss spectroscopy (EELS)) was also used to determine the structural characteristics and the composition of individual nanowires. For TEM observations, the as-grown nanowires were first removed from the substrate by ultrasonic

and then suspended into ethanol, finally dispersed onto TEM copper grids with carbon supporting film. Cathodoluminescence (CL) spectra were recorded at room temperature in an ultrahigh-vacuum SEM (Quanta 400 FEG).

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RESULTS AND DISCUSSION Figure 1b is a low-magnification SEM image taken from an area where the tungsten filament is contacted closely with the substrate (as illustrated in position A of Figure 1a) and shows the morphology of synthesized products. As can be seen, a white circle exhibits at the edge of the substrate. To clarify the white circle, a magnified SEM image was taken, as shown in Figure 1c, where a large amount of straight one-dimensional nanostructures was seen. Figure 1d is a close-up SEM image and shows that the 1D nanostructures are nanowires with diameters of 100−200 nm and lengths of 5−10 μm in a relatively uniform distribution. However, in the area far away from the tungsten filament (as illustrated in position B of Figure 1a), another distinguished structural characteristic can be clearly seen in Figure 1e, showing a high yield of long and curved nanowires. The magnified SEM image (as shown in Figure 1f) shows that such nanowires have diameters of 80− 150 nm and a length of tens of micrometers. To understand the detailed structural characteristics of the synthesized straight nanowires, we performed TEM investigation. Figure 2a is a bright-field TEM image of a typical nanowire and shows that the nanowire has a straight B

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Figure 2. (a) Low-resolution TEM image of one single nanowire. (b) BF image of the nanowire tip, showing a black hemisphere-like dot embedded in the tip of nanowire; the inset is its corresponding HRTEM image. (c) HAADF of the tip of nanowire, showing the hemisphere-like dot consisted of heavy atoms. (d) EDS spectra of the hemisphere-like dot and nanowire body. (e) EELS spectrum of the synthesized straight nanowire.

characteristic with a diameter range from the root of ∼200 nm to the tip of ∼85 nm and the length of ∼6 μm. Figure 2b is a bright-field scanning transmission electron microscopy (STEM) image, where a dark hemisphere-like dot embedded in the tip of the nanowire can be seen. To evaluate the nanowire composition distribution, we took the high-angle annular dark-field (HAADF) images because compositional variations can be directly reflected in the HAADF images. Figure 2c is an HAADF image of the nanowire tip and shows a brighter hemisphere-like dot, indicating that the dot must contain heavier atoms. To confirm its composition, we applied EDS analysis, and the results were shown in Figure 2d. Two

positions were collected with position 1 from the dot and position 2 from the wire. The quantitative analysis of EDS spectrum of the position 2 suggests that the nanowire is SiOx with x ≈ 1.6, and, from EDS spectrum of the position 1, the bright dot contains W (note that C and Cu is from the TEM grid). On the basis of these experimental results, we anticipate that the SiOx nanowires must be induced by the W-containing catalysts. High-resolution TEM (HRTEM) was also applied to determine the structural characteristics of nanowires. As shown in the inset of Figure 2b, the amorphous nature of the nanowire is revealed. Figure 2e is an EELS spectrum taken from the C

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Figure 3. (a,b) TEM, HRTEM, and EDS characterization of the nanowires grown at the area far from the tungsten filament, showing they are amorphous SiOx nanowires with curved and tangled morphology.

Figure 4. Schematic diagrams of growth mechanism: (a) WOx clusters formed on the substrate; (b) adsorption of Si and O to the catalyst; (c) formation of SiOx clusters and surface diffusion to interface between substrate and catalyst; and (d) the final state of formed nanowire.

nanowire, agreeing very well with the reference spectrum of amorphous SiOx,28 further confirming its amorphous nature. The microstructure of the long and curved nanowires was also investigated by detailed TEM investigations. Figure 3a is a bright-field TEM image of a typical nanowire, revealing its smooth surface and uniform diameter distribution along the growth direction. Extensive TEM observation of this type of nanowires indicates that no tungsten dots were found inside the nanowires. The inset of Figure 3a is a typical HRTEM image of such a nanowire, showing its amorphous nature. In addition, the EDS analysis was performed to reveal its composition, and a typical result is shown in Figure 3b, indicating that such type of nanowires is silica. To understand the role of Ge in the growth of straight nanowires, we repeated the experiment on a bare Si substrate with a tungsten filament for the nanowire growth under the identical annealing condition. After annealing, no nanowires were found on the substrate. Meanwhile, we also repeated the experiment on a substrate with Ge islands grown on its surface under the identical annealing condition. The Ge islands were

formed due to the strain relaxation between the Ge/Si lattice mismatch when depositing pure Ge on Si substrate.29 After annealing, a similar result was observed. These experimental results indicate that Ge indeed plays an important role in the nanowire growth. In fact, Ge may assist the generation of Si source for the SiOx nanowire growth.30 In the area far away from the tungsten filament, because of its very low vapor pressure in the growth temperature, the effect of tungsten on the nanowire growth is negligible. The formation mechanism of the long and curved SiOx nanowires in this area was attributed to the SLS growth model catalyzed by Ge.30 In the area near the tungsten filament, the nanowire formation mechanism notably cannot be explained by the SLS growth mode because of the different morphology of nanowires grown near and far from the tungsten filament. On the basis of the morphological differences, we anticipate that the tungsten is responsible for the growth of straight nanowires. Their formation should be attributed to the synergetic effect of the tungsten filament and Ge on the substrate. The fact of the hemisphere-like dot embedded in the nanowire tip indicates that the straight D

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Figure 5. Schematic diagrams of the cross-section view of (a) the catalyst at top of a straight nanowire and (b) the catalyst on the bottom of the curved the nanowire. The insets are the corresponding TEM images.

absorb the additional reactive silicon oxide clusters. Because of the low solubility of SiOx in the catalyst,28 these SiOx clusters slipped to the bottom of the catalyst and penetrated below the catalyst detaching it from the substrate and forming a nanowire, which could be an energetically preferable interface compared with direct contact on the substrate and the catalyst,28 as illustrated in Figure 4b,c. The new deposited SiOx clusters continuously diffuse to the catalyst−SiOx interface instead of formation of new structure of SiOx on the catalyst surface (which will increase the system energy) to maintain a lower energy. The nanowires are therefore formed and grown under the condition of continuous supply of SiOx clusters, as illustrated in Figure 4d. As shown above, two different morphologies of SiOx nanowires coexisted on the substrate, that is, near tungsten filament showing a straight morphology and far-away area showing a curved morphology. The morphology differences, that is, the straight or curved feature, may be explained by the contact-angle anisotropy (CAA)36 or screw-dislocation mechanisms.37 In view of the amorphous nature of our nanowires, the screw-dislocation mechanism is impossible. Therefore, the CAA mechanism is proposed to explain the amorphous nanowires grown by catalyst, in which the straight or curved feature arose from the variations in the velocity of the growth front at the catalyst−nanowires interface depending on the contact angles anisotropy. The motion path of the catalyst could be quantified by the work of adhesion (W), which is related to the surface tensions between the nanowire-vapor (αSV), nanowire-catalyst (αSL), and catalyst-vapor (αLV). The probability of adsorption of new atoms at the catalyst− nanowire interface depends on W; that is, the lower W will generate higher adsorption probability. To reveal the growth mechanism of the straight and curved nanowires in our experiment, we examined the interfaces between the catalyst and nanowires in such two different nanowires by cross-section TEM carefully to confirm the contact angle. Figure 5a schematically depicts a cross-section view of a catalyst atop a straight nanowire, whose cross-section TEM image was shown in the inset. At the interface labeled A, the interfacial surface tension αSV and αSL are antiparallel, while the αLV is perpendicular to αSV and αSL. At the interface B, the interfacial surface tension has the same relationship. As a consequence, W has the following relationship:

nanowires must be catalyzed by tungsten or tungsten containing compound. It should be noted that tungsten has the highest melting point (3422 °C) and lowest vapor pressure in all pure metals,31 which make it difficult to evaporate into the atmosphere and to form tungsten containing alloy(s) in our synthesized temperature. Meanwhile, because, in our experiment, it is impossible to eliminate oxygen in the quartz tube, evidenced by the resulted SiOx nanowires, the tungsten filament should be inevitably oxidized. This can be certified by the fact that the color of the tungsten filament has been changed after the nanowire growth when compared with the initial color before the nanowire growth. Furthermore, it has been well-documented that tungsten, especially its nanostructures, can react violently with oxygen at temperature above 400 °C.32,33 For this reason, the high temperature in our experiments should generate a violent oxidation reaction when the oxygen encountered the tungsten filament. Because of insufficiently oxygen, a tungsten suboxide, WOx, should be generated with emitting large amount of heat, which will consequently result in a local combustion, and the local temperature can reach the range of 1453−1538 °C.32,33 Such a high temperature is sufficient to timely evaporate the WOx from the tungsten filament, and the resulted WOx clusters can be melted (a melting point of 1473 °C in its bulk form). In addition, it has been reported that WOx can be acted as the catalyst and simultaneously provide the sources for the tungsten oxide nanowire growth.34 Taking these facts and our experimental evidence into account, we anticipate that the unsaturated WOx should be the catalyst for our SiOx nanowire growth. The WOx species are expected to condense readily from the vapor form onto the substrate in an amorphous state,34 which, as a catalyst, would absorb the ambient gas species, as shown in Figure 4a. On the GeSi substrate, the GeSi alloy droplets should be formed on the substrate because the synthesized temperature was higher than the eutectic point of the GeSi alloy (938 °C). In fact, we have found that a large number of particles were formed on the surface of the GeSi alloy film. Meanwhile, the liquidized GeSi alloy would evaporate at such high annealing temperature.30 It should be noted that large quantities of pits were left on the GeSi alloy substrate, which should be the trace of the GeSi alloy evaporation.30 When colliding with the oxygen, oxidized Ge or Si gas species should be expected. However, a preferential oxidized Si will take place because of the higher binding energy of Si−O relative to the Ge−O.35 Now, the WOx condensation will act as nuclei to

WA = αSV + αLV − αSL = αLV = WB E

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Figure 6. (a) SEM image of the synthesized straight SiOx nanowires. (b) CL spectrum taken from one spot of one single nanowire. (c) Corresponding CL emission image using emission peak.

CL spectroscopy combined with SEM imaging has been proven to be a powerful tool for probing the local characteristics of the nanostructures due to its high spatial resolution. CL spectroscopy was employed to detect and analyze the optical property of the synthesized nanowires.39 Figure 6a is a typical SEM image showing two straight nanowires, and Figure 6b shows a typical CL spectrum recorded from one spot of a single nanowire indexed by a red square, as shown in Figure 6a. One strong but broad emission peak located at 460 nm (2.7 eV) can be seen clearly. Nishikawa et al.40 observed several luminescence bands in various high purity silica glasses, with different peak energies ranging from 1.9 to 4.3 eV under 157 nm laser. It reveals that the 2.7 eV peak is ascribed to the oxygen deficiency. Considering our growth conditions, the oxygen was unintentionally introduced into the annealing furnace. Therefore, it is believed the high oxygen deficiency should be responsible to the CL emission in our nanowires. Figure 6c is the corresponding CL emission image recorded from 460 nm peak and clearly shows the homogeneous emission of the nanowires. Overall, the blue room-temperature visible luminescence in our nanowires provides the potential application of these nanowires to blue intensive blue-light emitters for use in light source for lamps based on Si technology.

Therefore, the uniform work of adhesion will generate a symmetric growth, that is, straight growth. Meanwhile, a schematic illustration of cross-section view of a catalyst at the bottom of a curved nanowire is shown in Figure 5b, and the corresponding TEM image was shown in the inset. At interface A, the interfacial surface tensions αLV and αSL are antiparallel, while αSV has an angle θ1 with αLV and αSL. At interface B, the contact angle (θ2) is different with θ1. The work of adhesion WA can then be defined as follows: WA = αSV + αSL − αLV = αSV + αSL − (αSL + αSV cos θ1) = αSV(1 − cos θ1)

Correspondingly, WB = αSV(1 − cos θ2). Therefore, the asymmetric work of adhesion, that is, WA ≠ WB, will make the nanowire growth asymmetric, that is, curved growth. It is of interest to note that the straight nanowires are slightly tapered, with the base diameter larger than the tip. Admittedly, a tapered morphology with a wider base is more stabilized than the wire with uniform diameter. In general, the tapered nature of the nanowires is attributed to the lateral growth of nanowires.38 It is believed that the tapering of our straight nanowires is caused by the deposition of SiOx on the sidewall of the nanowires possibly through their diffusion from the Si substrate to the catalyst or direct deposition of SiOx from the vapor. F

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CONCLUSIONS In summary, a large number of straight SiOx nanowires were achieved by annealing GeSi alloy film sample with tungsten in a quartz tube furnace with forming gas at high temperature. By comprehensive TEM and EDS characterizations on systematically designed comparative experiments, a WOx-catalyzed contact angle anisotropy growth model was proposed for the growth of straight SiOx nanowires. These unique straight amorphous nanowires demonstrate homogeneous blue roomtemperature visible luminescence, which provide an alternative candidate as blue intensive blue light emitters and scanning near-field optical microscopy tips.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-21-65643827; E-mail: [email protected] (Z.M.J.). Phone: 61-7-33463195; E-mail: [email protected] (J. Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the special funds for the Major State Basic Research Project (No. 2011CB925601) of China, and the Natural Science Foundation of China (NSFC) under Project No.61274016, and the Australian Research Council. The Australian Microscopy & Microanalysis Research Facility, established under the Australian Government’s National Collaborative Research Infrastructure Strategy, is gratefully acknowledged for providing access to the facilities used in this work.

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