2004 4th IEEE Conference on Nanotechnology
Catalytic syntheses
of
silicon nanowires and silica nanotubes
Yi-Han Yang, Sheng-Jia Wu, Hei-Shan Chiu, Ping-I Lin and Yit-Tsong Chen
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, ROC and Institute
of
Atomic and Molecular Sciences, Academia Sinica, P.O.
Box
23-166, Taipei 106, Taiwan, ROC
Absi"-
Silicon nanowires (SiNWs) have heenfabricated with metal- and Si0,satslyses assisted by laser ablation. In the growths of single-crystalline SiNWs by metal -catalysts, it is found that SMWs are along <111> growth direction; however, the <112> growth direction is resulted in the Si02-catalyzed SiNWs. Pressure effect on the longitudinal and transverse growing rates of these SiNWs has also been examined. With the chemical vapor deposition method, the silica nanotuhes (SiOtNTs) could he achieved
with Au nanoparticles as catalysts. From electron dillraction pattern, it shows that the Si02NTs are amorphous and the possible growth mechanism would he discussed.
Index Terms - chemical vapor deposition, growth mechanism, laser ablation, pressure efiecf silica nanotuhes, silicon nanowires, vapor-liquid-solid (VLS) growth.
I. INTRODUCTION
The optical and electrical properties of semiconductor- based nanostructures have recently stimulated tremendous research interest. A distinctive feature for semiconductor- based nanostruchres relative to bulky material is the well- known quantum confinement effect owing to the reduced size a n d o r dimensionality
[I].
Silicon (Si) is an important semiconductor material with its contemporary microelectronic technology being one of the greatest successes for the past century. Nevertheless, there is still room for improvement in the applications of Si-based optoelectronic devices. The advance of nanotechnology has led a way to fabricate semiconductor materials from micro- to nanometers in size. Among other developments. more and more syntheses for one-dimensional nanowires have been accomplished. For Si, in particular, intensive research of one-dimensional nanowires and their corresponding optical and electrical properties have been studied by several groups. In the researches approached by these groups, the wirelike crystalline nanostructures of Si were often fabricated via vapor-liquid-solid (VLS) growth[Z].
The sizes of the catalysts are considered to be responsible for diameters ' of the resultant silicon nanowires (SiNWs). Based on this correspondence, the application of laser ablation to vaporize a target sample of bulky Si, mixed with metal catalyst, and to form small sizes of nanoclusten in the initial VLS process is a uniqueand quite powerful method in the fabrication of one- dimensional SiNWs.
Besides the SiNWs for fabrication, the silica nanotubes (Si02NTs) are also produced by the chemical vapor deposition method. Among the oxidic instances, particular focus has been put on nano-framed silica because of its potential applications as a catalyst support material [3], as
a core material for biosensors and biomarkers [4], as storage/delively containers for biochemical substances [ 5 ] , and in optoelectronic nanodevice design [ 6 ] . Until now, most of the Si02NTs are synthesized with the soft
chemical methods.
Here, we will investigate the catalytic roles of different metal catalysts (Fe, Ru, Pr, RuCI,, and Pr6OI1) and Si02 in the syntheses of SiNWs assisted by laser ablation. From these comparisons, we have tried to discem the growth mechanism via Si02-catalysis from that via the VLS in traditional metal-catalytic processes. Differences in the crystalline and wire growth axes of SiNWs have been examined for the S O 2 - and metal-catalytic SiNWs.
11. EXPERIMENT
The apparatus used in our catalytic-growth experiments assisted by laser ablation is described as follows. An evacuated alumina tube, placed inside a furnace, contains a target sample of Si powder and metal catalyst. We have used iron (Fe), ruthenium (Ru), praseodymium (Pr), ruthenium chloride (RuCl,), and praseodymium oxide (Pr6OI1) powders, respectively, as metal catalysts. Altematively, Si02 was regarded as another supplying
source for
Si,
and had also been introduced in the target sample. Totally, we have carried out six experiments, including %+Fe, Si+Ru, Si+Pr, Si+Si02+Fe, Si+Si02+RuC13, and Si+Si02+Pr6011 ' as reactingingredients, respectively. In the experiments without Si02
in the target sample, Si powder and metal catalyst (Fe, Pr, or Ru) were mixed with a weight ratio of 90 : 10%. When Si02 was used in the growth of SiNWs, the wt. ratio of Si :
Si02 ; metal compound (Fe, Pr6011, or RuCI,) was typically 45 : 45 ; 10%.
While in the Si02NTs case, a bubble bath container is placed with few amounts of SiCh (- 50 ml), which would
TABLE I
Growth Direction, Composition, Crystallization, and Dimension of Catalytically Synthesized SiNWs reacting ingredient
%+Fe Si+Ru Si+Pr Si+Si02+Fe" Si+Si02+RuCl,b Si+Si01+Pr6011c
_ _
Crystalline gowth axis
[i
I
i]
11
i
11
[1111
[ i i i ]
[iii]
[I
i
11
Wire growth axis
[1111
- _[ i i i j
[1111
_ -[ii21
[iiq
[iiq
Tip composition FeSi2 RuSi, PrSi, SiO. SiO, SiO.
Wire crystallization single single single + poly single single single
Si core diameter
-
36 nm-
5 nm-
16nm
- 8 m - 5 n m -15nmSiO, sheath thickness
-
5 nm-
9 nm-
3nm-
- 9 n m - l o @ - 5 n m a Ratio of 92 : 8% is found for the fabric_ated SiNWs with[I 1 11
crystallinego_wth axis and [ 1121
wire growthaxis catalyzed by S O 2 to those with
[
1 111
crystalline growth axis and [ 1 1 I ] wire growth axis catalyzed by Fe.This column represents the major product of SiO,-catalytic S i m s .
Ratio of 92.3 : 7.7% is found for the fabdcated SiNWs with
[l 1 11
crystalline growth axis and [TI21 wire growth axis catalyzed by Si02 to those with[I
111
crystalline growth axis and[I 1 I]
wire growth axis catalyzed by RUC&. This column represents the major product of SiOl-catalytic SiNWs.Ratio of 95.5 : 4.5% is found for the fa_bric_ated SiNWs with
[I 1 I]
crystalljnygowth axis and [TI21 wire growth axis catalyzed by SiOz to those with[ 1 1 I ]
crystalline growth axis and [1
1 I ] wire growth axis catalyzed by Pr6OI1. This column represents the major product of Si02-catalytic SiNWs.be the source material of the reaction. The carrier gas used here is the hydrogen gas, and with controlled for its flowing rate of
-
750 sccm. With the bubbled system, theS i c 4 is carried out by the carrier gas and goes into the reaction chamber under the heating of furnace. The temperature
for
reaction used for SiOzNTs synthesis is about 65OOC in the center, and while the vapor from the bubbled system would start to deposit onto the silicon substrate, which is coated with a thin layer (-10-20 nm) of Au. Each experiment for Si02NTs reaction is-
20 mins and keeps the chamber pressure of-
100 Torr, and then the chamber is vacuumed to about 30 mTorr to pump out the reaction species. When the temperature decreases to room temperature, the silicon substrate can be taken out and a very light white color can be seen on the silicon substrate.111. RESULTS AND DISCUSSION
A . Silicon Nanowires Synthesis
As previous saying, the fabrication of SiNWs can be divided into six groups for discussion. In the Fe group (Si+SiOl+Fe, Si+Fe), it could he first found that the wire growth directions of the two catalysts are very different. According to high-resolution transmission electron microscopy (HRTEM) images, the wire growth direction of Si02-catalytic SiNWs is [i12] while it becomes to f i l i ] in the pure Fe-catalyzed SiNWs case (Fig. l a and Ib).
Such observation would be also investigated in the others, Ru (Fig. IC and Id) and Pr (Fig. l e and
IQ
groups, that the wire growth axis would be <112> in Si02-supplied precursors hut the < I 1 I > wire growth direction is formed in pure metal catalytic SiNWs.Fig. lI (a) HRTEM image of the SiO,-catalytic SiNW showing the (1 11) lattice plane with d-spacing of 0.32 nm while the wire
growth axis is [il2]. @) HRTEM image of the Fegalytic SiNW, which indicates that the wire growth axis is [ I l l ] . (c) HRTEM image of the SiO,-catalytic SiNW with the core diameter -5 nm shows the wire growth direction of [1121. (d) HRTEM image of the Ru-catalytic SiNW shows the cry2talline
( i i i ) planes are parallel to the wire growth axis of
p
I I]. (e) HRTEM image of the Si02-catalytic SiNW shows the [ I 121 wire growth direction.(9
SEM morphology of the Pr-catalytic SiNWs are with -30 nm of diameter and contain metal-tips (white spots), which provide the evidence of typical VLS growth mechanism.Not only the wire growth directions are different, with detailed comparisons, it shows that the growth mechanism could he realized as two different models. For pure metal- catalyzed SiNWs, the tip composition shows that it forms silicides and could be believed the SiNWs follow the typical VLS process of growth. However, for SO2- catalytic SiNWs, such silicides composition could not be found. In the study of Si growth on SiOz Bubstrate by chemical vapor deposition, Dimitriadis et al. [7] found that an orientation filtering mechanism, due to the growth- velocity competition in the early stage of growth, it is responsible for the preferred orientation of Si films.
B. Silicu Nanotubes Synthesis
The SEM and TEM morphologies of the CVD-grown Si02NTs can he seen in Fig. 2. These SiOzNTs contains only the silicon and oxygen elements purely by EDS
measxement and exist the ratio for Si : 0 = 1 : 2, which means that the composition would be the SiOz nanotubes. Furthermore, it is hard to find the Au-tip in the end of
SiOZNTS
in
theSEM
image, implying that these Si02NTs would not grow via the “tip growth” for normal VLSmechanism. Another important point here is that the sheaths of these SiOZNTs are so heavy for its thickness, and almost equal sizes for both the sheaths and tube channels.
Fig. 2. (a) SEM image of SiOzNTs, which are with uniform diameter of 40-60 tun and the length could extend to tens of pm.
@) SEM morphology of SiOzNTs with thin thickness of sheaths
(-10-20 nm) compared with -3W80 nm of core diameter. (c) TEM image of the Si02NTs with thick sheaths, showing that the core and sheath diameters are almost with equal sizes. (d) TEM morphology of the thick-sheath Si02NTs. A catalytic Au tip is contained with ‘polygonal” shape and with further check is found that the Si02 sheath species are diffused from the Au tip and run out from the Au{ I I I } facets to form the Si02NTs.
IV. CONCLUSION
The growths of crystalline SiNWs with the catalyses of pure metals (Fe,
Ru,
and Pr) and S O z . Si { 11 1) planes have been found to be most stable in these catalytic fabrications. While the wire growth axis is along < I l l > for the pure metal-catalytic SiNWs complying with VLS mechanism, the wire growth axis directs to cl 12> in the SiNWs catalyzed by SOz. Under the chemical vapor deposition process, the amorphous SiOzNTs can be formed by the diffusion from the Au{ 11 1 ) facets.ACKNOWLEDGEMENT
The electron microscopy measurements for this study in Instrumentation Centers of National Taiwan University and National Tsing-Hua University are acknowledged.
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