Hydrothermal Deposition of Silicon Nanochains

Si nanochains have been prepared via a simple hydrothermal deposition route using silicon monoxide as the starting material at 300◦C. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrum (EDS) and high-resolution transmission electron microscopy (HRTEM) are used to observe the structure and morphology of the products. The obtained Si nanochains are composed of knots and necks with polycrystalline silicon cubic structure. The surface of the Si nanochains is covered with amorphous silica outer layers. The growth mechanism of the Si nanochains is discussed. [DOI: 10.1380/ejssnt.2008.171]


I. INTRODUCTION
Hetero-structured Si nanochains have peculiar onedimensional nanostructures including crystalline Si nanospheres connected by necks of amorphous Si oxide at a nearly equal spacing and were first reported by Kohno and Takeda [1].They exhibit excellent potential application in single electron and opto-electronic devices owing to the alternate semiconductor/insulator structure and used the nanochains as templates for fabricating more complex nanostructures [2,3].Therefore, great interest has been devoted to the research on the Si nanochains.Crystalline Si nanochains had been prepared by a self-organized process via an extension of the vapor-liquid-solid mechanism using gold as catalyst at the temperature of higher than 1200 • C [1,[4][5][6].The physical properties of Si nanochains, such as plasmon excitation [6], phonon structure [7], photoluminescence and electron transport [8] had also been researched.In addition, Lee et al. [9] obtained a small amount of silicon nanochains besides a large amount of Si nanowires with the diameter of 3-43 nm and length of several dozens to several hundreds of micrometers by laser ablation method at the temperature of 1200 • C. The authors consider that the structure can be explained by polycenter nucleation process.Periodic unstable growth process makes the diameter of the Si core in the nanochains change regularly.Boron-doped Si nanochains had also been prepared by laser ablation of silicon monoxide powder mixed with B 2 O 3 powder at the temperature of 1200 • C [10].
According to the present literatures, the preparation temperature of Si nanochains is high (higher than 1200 • C in general) and these methods make this approach complicated and expensive.Therefore, the low temperature growth of Si nanochains and development of less expensive synthesis methods are of great interests for technological applications.Hydrothermal method has shown the potential to synthesize one-dimensional nanoscale materials at relatively low temperature [11][12][13][14][15].In this paper, we report a simple hydrothermal deposition route for the synthesis of Si nanochains without metallic catalysts or doped elements at 300 • C. The products have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectrum (EDS).The growth mechanism of the obtained Si nanochains is also discussed.

II. EXPERIMENTAL
The preparation of Si nanochains was performed in an autoclave.The autoclave is composed of stainless steel (1Cr18Ni9Ti).The main detail of the autoclave is shown as follows: Maximal pressure is 22 MPa, maximal temperature is 500 • C, volume is 1000 mL, power is 1.5 kW and stirring velocity of the stirrer is 0-1000 r/min.1g SiO powder (purity: 99.99%, particle size: ∼73 µm, density: 2.1 g/cm 3 ) was mixed with 49 ml distilled water.Then the mixture was put into the autoclave.After the autoclave was sealed safely, it was heated to 300 • C of temperature, 5.7-6 MPa of pressure, 100 rpm of the rotating speed for the stirrer of the apparatus and the temperature and pressure were maintained for 24 h.Subsequently the autoclave was cooled naturally.Finally, the bulk white deposit was obtained from the stainless steel cooling tube in the autoclave after the experiment.
SEM observation was performed using JEOL JSM5410 SEM with a 15-kV accelerating voltage.Chemical analysis was performed with an EDS attached to the SEM.The product was separated from the stainless steel cooling tube using hardy plastic utensil.The surface of the SEM sample was treated by spraying Au in order to increase the electrical conductivity before the sample was observed by SEM.TEM and HRTEM samples were prepared by putting several drops of solution with Si nanochains onto a standard copper grid with a porous carbon film after the Si nanochain samples were dispersed into distilled water and treated for about 10 min using supersonic wave apparatus.TEM images were taken on a JEOL JEM-100CXII transmission electron microscope, operating at 80 kV.HRTEM observations were performed using JEOL JEM-2100 transmission electron microscope operating with a 200-kV accelerating voltage with a GATAN digital photography system.

III. RESULTS AND DISCUSSIONS
Bulk white deposit can be observed from the stainless steel cooling tube of the autoclave.Figure 1 is the photograph of the deposited sample.The image shows the typical feature of the sponge-like product designated by white arrow.SEM observations reveal an abundant member of nanochain structures.Figure 2(a) is the general SEM image of the deposited sample.The diameter of nanochains is about 100-200 nm.EDS spectrum of the deposited sample was measured in order to determine the product composition.Analysis using EDS (Fig. 2(b)) attached to the SEM shows that silicon and oxygen are detected, confirming that the deposited sample is made up of silicon and oxygen.A copper substrate was used in the SEM study.Hence Cu peak is observed in the spectrum.
The branch-like structure can be observed from the TEM image (Fig. 3).It can be seen that the outer diameters of nanoparticles are almost uniform.A silicon nanochain consists of knots and necks with approximately equal distance between them.The average diameter of the knots is about 150 nm.The average diameter of the necks is a little smaller than that of the knots, about 100 nm.The length of the nanochains is several micrometers, even up to tens of micrometers.The corresponding selected area electron diffraction (SAED) pattern is used to certificate the structure of the nanochains as shown in the inset of right-upper corner of Fig. 3.The polycrystalline rings are match well with the {111} and {220} crystalline plane families of silicon with polycrystalline diamond cubic structure.The HRTEM images (Fig. 4) reveal that the nanochains are well crystallized and interplanar spacing of the crystalline in the sample is about 0.31 nm according to the measurement and calculation by software of Digital Micrograph applied in the HRTEM, matching well with the {111} plane family of silicon.The crystalline with different direction can be distinctly observed.The knots and necks of silicon nanochains are connected with the same crystalline Si structures according to the HRTEM image.The results demonstrate that the nanochains are a kind of polycrystalline structure which is consistent with the SAED result.The surface of the silicon nanochains is composed of amorphous structures with the thickness of about 5 nm.The amorphous outer layers are same to that of Si nanochains prepared by other methods [1,9,16].EDS result of the sample (Fig. 2(b)) shows that element oxygen is observed besides the element silicon.Silica is the most stable compound of Si and O.So the amorphous structure is determined to be amorphous silica.The above results show that the obtained silicon nanochains are composed of knots and necks with relatively good polycrystalline structures.The knots and necks are connected with crystalline Si.The nanochains have semicircular closed structures and the surface of the Si nanochains is covered with amorphous silica outer layers.
The crystal growth by hydrothermal process includes some conditions [16]: (1) the autoclave would endure high temperature and high pressure; (2) There must be temperature gradient in the autoclave so as to accelerate the growth of the crystal; (3) The temperature coefficient of solubility is adequate.The solution has enough supersaturation degree under proper temperature.In the experiment, the autoclave was used as experimental container, which could endure 22 MPa and 500 • C.Under high temperature, the distilled water would expand in the autoclave, so the high pressure of crystal growth condition will simulate in the autoclave.Under high temperature and pressure, the disproportionation reaction and the solubility of silicon monoxide would increase.The temperature and pressure in the autoclave would be controlled by the http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology control instrument.The center is far from the heater, then the temperature gradient in the autoclave is formed naturally.
According to the above analysis, we propose the growth mechanism of the silicon nanochains, which is similar to the nucleation process of silicon nanowires with oxideassisted growth [17,18].Silicon oxide is believed to play an important role on the formation and growth of silicon nanochains.Si atoms and silica molecules form to be gas under the hydrothermal conditions by the disproportionation of the starting material SiO as follow.2SiO = Si + SiO 2 . ( Silicon nuclei form from gas Si atoms owing to the effect of the covalent bonds of Si.The growth possibility of the Si nucleus is same along different directions according to the thermodynamics theory.Therefore, silicon nuclei absorb a large amount of silicon atoms and silicon oxide from the atmosphere forming spheric structures after the initial silicon nuclei are formed (Fig. 5 from the mixture of Si and SiO 2 by the phase transformation [19] to form polycrystalline Si cores when the Si atoms in the nanoclusters are in the hypersaturated state. Polycrystalline Si cores absorb continuously the Si atoms and silicon oxide molecules in the atmosphere resulting in the formation of the nucleus with polycrystalline Si in the center and thin amorphous silica outer layers (Fig. 5(c)).
The amorphous silica plays an important role on the stability of the whole nanoclusters.The size of the nanoclusters is similar due to the equally stirring of the stirrer.The similar nanoclusters will incorporate with each other by the forces, such as van der waals force when the nanoclusters are close to each other (Fig. 5(d)).The collision opportunity of initial nanoparticles with silicon atoms and silica molecules increases due to the initial nuclei's uninterrupted movement with the continual stir by the magnetic stirring apparatus.A large number of silicon atoms bond with the initial nuclei and it increase the growth speed of nanoparticles.When the temperature in the autoclave was cooled to room temperature, the growth and link of silicon nanoparticles stopped.In the growth process, the silicon atoms incorporate into crystal lattice without catalyst participant and the structure of silicon nanoparticles is integrated with few defects.

IV. CONCLUSIONS
A simple and easily controllable hydrothermal deposition route has been used to prepare Si nanochains at 300 • C using silicon monoxide as the starting material.The preparation temperature is far lower than that of other methods.The obtained Si nanochains are composed of knots and necks with good polycrystalline structure and amorphous silica outer layers.This method is easily controlled, very convenient and effective.It may stimulate technological interest and also prospect many applications in material fields.
FIG. 1: Photograph of the as-prepared sample.

FIG. 2 :
FIG. 2: (a) SEM image of the sample.(b) The EDS spectrum of the sample.

FIG. 3 :
FIG. 3: TEM image of the sample.The inset in the rightupper is the corresponding SAED pattern.