简介概要

Synthesis of ultralong Si₃N₄ nanowires by a simple thermal evaporation method

来源期刊：Rare Metals2013年第2期

论文作者：Wen-Qi Wang Xiao-Ping Zou Nan-Jia Zhou Guang Zhu Gui-Ying Wang Zhen-Sheng Peng

文章页码：186 - 190

摘要：Large-scale vapor-solid synthesis of ultralong silicon nitride （Si3N4） nanowires was achieved by using simple thermal evaporation of mixture powders of active carbon and monoxide silicon. The products were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy. The results suggest that the silicon nitride nanowires have a smooth surface, with lengths of up to several hundreds of microns and diameters of 100-300 nm. A detailed study of both the chemical and structural composition was performed. Such ultralong silicon nitride nanowires demonstrate potential applications as materials for constructing nanoscale devices and as reinforcement in advanced composites.

Rare Metals 2013,32(02),186-190+2

Synthesis of ultralong Si₃N₄ nanowires by a simple thermal evaporation method

Wen-Qi Wang Xiao-Ping Zou Nan-Jia Zhou Guang Zhu Gui-Ying Wang Zhen-Sheng Peng

Anhui Key Laboratory of Spintronics and Nano-materials (Cultivating Base),Suzhou University

Research Center for Sensor Technology,Beijing Key Laboratory for Sensor,Beijing Information Science and Technology University

Department of Materials Science and Engineering,Northwestern University

Structure Research Laboratory,University of Science and Technology of China

作者简介：Zhen-Sheng Peng e-mail:ahpengzhsh1948@126.com;

收稿日期：24 September 2012

基金：supported by the Key Program of the National Natural Science Foundation of China(No.19934003);the Grand Program of Natural Science Research of Anhui Education Department(No.ZD2007003-1);the Natural Science Research Program of Universities and Colleges of Anhui Province(No.KJ2008A19ZC);the Opening Program of Cultivating Baseof Anhui Key Laboratory of Spintronics and Nano-materials(No.2012YKF10);

Synthesis of ultralong Si₃N₄ nanowires by a simple thermal evaporation method

Abstract：

Large-scale vapor-solid synthesis of ultralong silicon nitride (Si3N4) nanowires was achieved by using simple thermal evaporation of mixture powders of active carbon and monoxide silicon. The products were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy. The results suggest that the silicon nitride nanowires have a smooth surface, with lengths of up to several hundreds of microns and diameters of 100-300 nm. A detailed study of both the chemical and structural composition was performed. Such ultralong silicon nitride nanowires demonstrate potential applications as materials for constructing nanoscale devices and as reinforcement in advanced composites.

Keyword：

Si3N4 nanowires; Ultralong; Thermal evaporation;

Received： 24 September 2012

1 Introduction

One-dimensional nanomaterials are expected to play an important role in future in various applications due to their excellent mechanical,chemical,electronic,and thermal properties[1–8].In addition,with high dopant concentration similar to the III-N compounds(Ga N and Al N),the dopant-containing silicon nitride(thin films or large single-crystals)compounds are extensively used in the microelectronics industry for various applications as optical devices,such as dopant diffusion barriers,encapsulants for III–V semiconductors,and interlevel dielectrics[9–12].The state-of-the-art synthetic strategies toward silicon nitride nanostructure materials may be categorized into these three:(1)nitridation of silicon powders by a nitriding gas,usually N₂/H₂,mixtures or ammonia;(2)catalyzed reaction between silicon hydrides and ammonia;and(3)reduction of Si O₂,by the same gases[8].Mixtures of Si O₂?Si were also used as reacting powders[13].

In this work,we reported that the ultralong silicon nitride(Si₃N₄)nanowires were directly synthesized by thermal evaporation of the mixture powders of active carbon and monoxide silicon at 1,300°C without assistance of any metal catalyst.The as-obtained Si₃N₄nanowires have a smooth surface,and they are mostly 100–300 nm in diameter and up to several hundreds of micrometers in length.The nanowires are analyzed by various characterization methods.These results indicate that temperature and ambience are two key factors for the formation of Si₃N₄nanowires,and possible growth mechanisms are also discussed.

2 Experimental

The ultralong silicon nitride(Si₃N₄)nanowires were prepared in a horizontal electronic-resistance tube furnace with a gas supply and a control system.Figure 1 shows a schematic diagram of the experimental setup employed in this work[14].A p-type Si(111)wafer(90.0 mm 925.0 mm 9 0.5 mm)was used as substrate for the growth of nanowires.The wafer was ultrasonically washed in acetone for several minutes,and then rinsed with deionized water.The raw material was a mixture of active carbon and monoxide silicon powders with a weight ratio of 1:1.The source materials were loaded into a ceramic boat covered with cleaned Si wafer,and then the boat was transferred to the center of a ceramic tube mounted in the horizontal tube furnace with a diameter of 45 mm.Before heating,the tube furnace was purged with N₂gas for 10 min.Under ambient pressure and a constant flow of N₂(200 ml?min^-1),the furnace was heated to 1,300°C,and the temperature was maintained for 2 h.The samples were cooled naturally to room temperature under N₂atmosphere.

Fig.1 Schematic diagram of experimental setup

Fig.2 XRD pattern of as-synthesized products

The structure of the as-synthesized sample was characterized using a M21XVHF2Z(Mac Science Co.,Ltd.)X-ray diffractometer with Cu Ka radiation at room temperature.Further detailed structural information of silicon nitride nanowires was obtained by scanning electron microscopy(SEM)(Hatachi S4300),transmission electron microscope(TEM)(JEOL-2010),and selected-area electron diffraction(SAED).Compositional analysis was performed using an energy dispersive X-ray spectrometer(EDS).The specimens for TEM analysis were prepared by sonicating the samples in ethanol for 10 min,followed by drop-casting the suspension onto a microgrid covered with a holey graphite thin film.Raman spectrum was measured using a Bruker RFS100/S Raman spectrometer at room temperature.The 1,046 nm line of an Ar?laser was used as the excitation source.

3 Results and discussion

The XRD pattern of the products is shown in Fig.2.All diffraction peaks in the pattern can be indexed to hexagona a-Si₃N₄.The lattice constants calculated from the XRD data are a=0.7739 nm,c=0.56024 nm,which are in good agreement with those of the standard pattern(JCPDS Card No.41-1408).The absence of other peaks demonstrates the high purity of the products.

The morphology characteristics of as-grown Si₃N₄nanowires were examined using SEM and TEM.As seen from the low-magnification SEM image,as shown in Fig.3a,the sample consists of a large quantity of ultralong fibers with a uniform diameter.Analysis of a number of the fibers show that each has a diameter of about 300 nm,and a length of up to several hundreds of micrometers,while some of them even have lengths in the order of millimeters A high-magnification SEM image,Fig.3b,further suggests that the products clearly have uniform diameter and smooth surface without any particles.The EDS(attached to the SEM)spectra,as shown in the upper right inset in Fig.3b,confirms the composition of nanowires.

Fig.3 SEM image of as-synthesized products:a grown on substrate,b high-resolution SEM image in a and inset being EDS spectrum of product

Fig.4 TEM and HRTEM images of a-Si3N4nanowires:a low-magnification TEM of a-Si3N4nanowires and inset being its EDS pattern,b HRTEM image of a-Si3N4nanowire with[011]growth direction and inset showing its ED pattern,c TEM image of simultaneous growth of Y-junction Si3N4nanowires and inset showing its ED pattern,d HRTEM image of a-Si3N4nanowires with[200]growth direction taken from black rectangular area marked in c

Low-magnification TEM shows that there are straight wire-like structures,all with uniform diameters(as shown in Fig.4a).The inset in Fig.4a is the corresponding EDS(attached to the TEM)spectrum,which further confirms that these nanowires are composed of Si and N.It should be noted that the Cu peaks come from the TEM grid.Figure 4b shows an HRTEM image of one wire,in which the lattice fringes of the{100}has a d-spacing of 0.66 nm for the hexagonal a-Si₃N₄,which can be clearly seen.The corresponding ED pattern(inset in Fig.4b)can be indexed as the[010]zone axis diffraction pattern of the a-Si₃N₄single crystal.The out-of-focus diffraction pattern also suggests that long axis direction or the growth direction occurs along the[011]direction of the a-Si₃N₄crystal.Figure 4c is the TEM image of the simultaneously grown Y-junction Si₃N₄nanowires having a smooth surface and a uniform diameter of 30 nm.The corresponding selectedarea diffraction pattern of the Y-junction Si₃N₄nanowires(inset in Fig.4c)confirms that they are a-Si₃N₄single crystal.Figure 4d shows the HRTEM image of the black pane area marked in Fig.4c,which indicates that Si₃N₄nanowires with[200]growth direction is a structurally uniform single crystal,while no dislocations or other planar defects are observed.The edge of the wire is clean and very abrupt on an atomic scale,and there are no amorphous layers covering the surface.

Raman spectrum of a-Si₃N₄nanowires(as shown in Fig.5)were obtained at room temperature with an excitation wavelength of 1,046 nm(Ar?laser).It is well known that Raman scattering provides a particular insight into the microstructure and conformation of materials.An asymmetrical peak is discernable at around 3,186 cm^-1Meanwhile,no other impurity peaks are observed in the Raman spectrum,which confirms that high-purity Si₃N₄nanowires can be fabricated by this method.

Fig.5 Raman spectrum of the as-synthesized a-Si3N4nanowires

To better understand the growth process of a-Si₃N₄nanowire,we performed some comparative experiments to identify the effects of synthesis parameters on the nanowire growth,specifically,temperature,time,and the flow of carrier gas.At a temperature below 1,300°C,the reactive rate is very slow,and it is difficult to obtain the products.Therefore,1,300°C was selected as the growth temperature in this study.Owing to the slow forming rate of aSi₃N₄,it is necessary to have enough reaction time.Experimental results show that the reaction time of around2 h is optimal under the conditions used in this study.In addition,we have also performed additional experiments using higher flow rate of carrier gas(about 300 ml?min^-1),with other experimental conditions being identical.The result shows that large quantities of very thin and wide nanobelts with smooth surface are obtained(as shown in Fig.6a).The EDS spectrum(inset in Fig.6a)displays that these nanobelts are composed of Si and N.Figure 6b is TEM image of as-obtained products,which indicates that these nanobelts are a-Si₃N₄single crystal.Therefore,we speculate that the formation of nanobelts is strongly related to the higher flow rate of carrier gas.

Vapor–liquid–solid(VLS)[15]and vapor–solid(VS)[16]are the main growth mechanisms involved in the growth of 1D microstructures materials.Since no droplets are observed on the end of a-Si₃N₄nanowires from TEM images,whereas the screw-growth morphology of a-Si₃N₄nanowires is found,as shown in Fig.4a,the growth of a-Si₃N₄nanowires can be explained in terms of the VS mechanisms[13].The chemical reaction formula is expressed as follows[17]:

The growth process of a-Si₃N₄is related to the formation of nuclei of a-Si₃N₄and the 1D growth of the nuclei through the above reaction.The nuclei of silicon nitride are formed through reaction and grow in the 1D direction,which may be attributed to the confining action of the nanoparticles on the surface.Since no additional metal catalyst was used,high-purity samples were obtained during this process,which can be confirmed using XRD and Raman spectra(as shown in Figs.2,5).Meanwhile,the morphology and structure of samples will change from wire to belt with the increase of carrier gas flow rate(as shown in Fig.6).

4 Conclusion

The ultralong a-Si₃N₄nanowires were synthesized using thermal evaporation of the mixture powders of active carbon and monoxide silicon at 1,300°C without assistance of any metal catalyst.By means of SEM,TEM,and HRTEM these nanowires have a smooth surface,with uniform diameters of 100–300 nm and lengths of up to severa hundreds of micrometers.Synthesis temperature and the flow rate of carrier gas are crucial to the formation of a-Si₃N₄nanowires.The structural uniformity of these ultralong a-Si₃N₄nanowires could make them useful as a specific reinforcement in nanodevices design and fabrication,and this synthetic method demonstrates promising future for commercialization.

Fig.6 SEM and TEM images of a-Si3N4nanobelts:a SEM image and inset being EDS pattern,b TEM image,upper right inset showing its ED pattern,and bottom left inset showing corresponding HRTEM