刻蚀压强对制备碳纳米管形貌的影响

采用等离子体增强化学气相沉积技术,以C2H2、H2和N2为反应气体,制备出碳纳米管薄膜,利用扫描电镜(SEM)对其进行表征。结果表明:催化剂刻蚀压强对碳纳米管薄膜的生长起着重要作用,获得定向性好、密度适中、杂质缺陷少的碳纳米管的最佳的刻蚀压强为200 Pa。     

刻蚀时间对碳纳米管形貌的影响

采用等离子体增强化学气相沉积技术,以C2H2、H2和N2为反应气体,制备出碳纳米管薄膜。利用扫描电镜(SEM)和拉曼光谱仪对其进行表征,研究刻蚀时间对碳纳米管形貌的影响。结果表明:催化剂刻蚀时间对碳纳米管薄膜的生长起着重要作用,刻蚀时间为10 min时可获得定向性好、密度适中、杂质缺陷少的碳纳米管。     

氢气流量对碳纳米管生长的影响

采用等离子体增强化学气相沉积技术,以C2H2、H2和N2为反应气体,制备出碳纳米管薄膜。利用扫描电镜和拉曼光谱仪对其进行表征。结果表明:氢气流量对碳纳米管薄膜的生长起着重要作用,获得分布均匀、密度适中、杂质缺陷少的碳纳米管的最佳氢气流量为30 sccm。     

用ECR-CVD方法制备定向碳纳米管

以FeaO4纳米粒子为催化剂,CH4和H2为气源,采用电子回旋共振微波等离子体化学气相沉积技术(ECR-CVD)在多孔硅基底上制备出定向生长的碳纳米管.研究了气氛组成、气压、温度和反应时间对碳纳米管生长特性的影响.使用扫描电子显微镜(SEM)、透射电子显微镜(TEM)和拉曼光谱(Raman spectrum)表征了样品的形貌和结构.结果表明:气氛组成和气压影响了反应腔内离解碳的浓度,从而影响碳纳米管的成核、生长速度及定向生长;温度的变化改变催化剂的尺寸从而改变碳纳米管的直径,在过低的温度下碳纳米管不能实现定向生长;碳纳米管随着反应时间的延长而不断增长,但超过一定时间后催化剂颗粒被碳包覆而失去催化作用,生长停止.     

氢气流量对定向碳纳米管生长的影响及其生长机理研究

采用催化热解法,以二茂铁和二甲苯分别作为催化剂和碳源,在不同H2流量下,直接在硅基底上生长定向碳纳米管阵列.采用扫描电子显微镜(SEM)、透射电子显微镜(TEM)和拉曼光谱(Raman)对样品进行观察和表征,并研究了H2流量对定向碳纳米管生长的影响和碳纳米管定向生长的机理.结果表明:H2在反应过程中起刻蚀作用,H2流量为60cm3/min时,生长的碳纳米管定向性最好.     

预置镍化学气相沉积法制备定向碳纳米管(英文)

采用化学气相沉积法(CVD),在溅射了镍薄膜的硅基底上制备了定向碳纳米管薄膜。对镍薄膜的氨气预处理过程及其机理进行了研究。结果发现预处理后的岛状区域随着薄膜厚度的增加而增加,纳米粒子区域的变化则与之相反。对5nm的镍薄膜进行预处理能获得细化和均匀分布的纳米粒子,有利于定向碳纳米管的生长。碳纳米管的生长过程及其细微结构与温度有很大关系。碳源的分解、碳原子在催化剂内部的扩散以及催化剂粒子的团聚三者之间的竞争决定了碳纳米管的生长情况。本文分析了碳纳米管的顶部生长模式及该模式下催化剂粒子的形态变化。     

定向碳纳米管薄膜的制备

以二茂铁为催化剂前驱体,氢气为载气,乙炔为碳源,硅片作衬底,用化学气相沉积法,采用不同的催化剂引入方式,在700℃下分别制备出定向碳纳米管薄膜及非定向碳纳米管薄膜.并基于实验结果对影响生长定向碳纳米管的因素进行分析,表明催化剂颗粒的诱导作用是导致生长定向碳纳米管的重要原因.     

ECR-CVD方法生长a-SiNx:H薄膜的研究

使用微波电子回旋共振等离子体化学气相沉积(ECR-CVD)方法室温生长了非晶氢化的氮化硅薄膜,通过改变前驱气体(SiH4 80%Ar和NH3)的流量比,研究了薄膜的生长速率、等离子体的发射光谱和薄膜的红外特性.结果表明:随着NH3流量的增加,氮化硅薄膜的生长速率呈下降趋势,这主要是由于等离子体中的气相前驱成分之一硅基团浓度的不断下降所导致的;随着NH3流量的增加,薄膜中键合了较多的具有较高电负性的N原子是Si-N和Si-H伸缩振动发生蓝移的主要原因.红外光谱的定量计算表明所制备的氮化硅薄膜具有相对较低的H浓度,约15%左右.文中对氮化硅薄膜的生长机制也进行了讨论.     

多晶Si薄膜的低温生长及其表面反应的控制

本文报道用反应气体SiF4和H2的微波等离子体化学气相沉积法低温(360℃)生长多晶Si(poly-Si)薄膜及其生长表面反应控制.实验发现,生长压力对晶粒的结晶取向有很大影响.改变SiF4与H2的流量比以选择等离子体中的活性集团,并结合外加偏压抑制带电粒子对薄膜生长表面的轰击是控制生长表面反应、制备高质量poly-Si薄膜的有效方法.用这种方法制备了H含量低达～1.0at.%、拉曼特征峰半高宽仅为～4.4 cm-1的poly-Si薄膜.     

衬底温度对碳纳米管生长和结构的影响

用CH4、NH3和H2为反应气体,利用等离子体增强热丝化学气相沉积在沉积有Ta缓冲层和Ni催化剂层的Si衬底上制备了准直碳纳米管,并用扫描电子显微镜和透射电子显微镜研究了它们的生长和结构随温度的变化.结果表明生长的准直碳纳米管是竹节型结构,其直径随衬底温度的降低而减小,生长速率随衬底温度的升高有一极值.从催化剂在衬底温度作用下的变化开始,分析了衬底温度对碳纳米管生长和结构的影响.     

Ni催化剂在N2O／N2／NH3中退火对碳纳米管表面结构及场发射特性的影响

研究在N2O/N2/NH3氛围中对Ni催化剂进行退火处理,旨在探讨退火处理对所生成碳纳米管的表面结构及其发射特性的影响.从表面结构及表面元素分析结果发现:Ni催化剂在N2O/N2/NH3氛围中退火处理之后,Ni催化剂的颗粒大小及催化剂的化学成分发生改变,进而影响所合成的碳纳米管的表面结构及场发射特性.扫描电镜显示:经过N2O退火前处理后,催化金属薄膜在成核时较易形成均匀性的金属颗粒,且金属颗粒较小.比较经N2O/N2/NH3氛围退火处理之后所合成的碳纳米管结果,经过N2O前处理可以有效抑制非品质碳的成长,使所成长出的碳纳米管数量最多、场发射电流最大.原因主要是因为N2O对催化剂镍膜金属前处理过程中分解出的氮原子及氧原子会活化及氧化催化剂Ni金属,并使所形成的Ni金属颗粒较小且更为均匀,造成表面型态上的显著改变,有助于使合成的碳纳米管场发射电流变大.     

镁-镍合金催化剂制备及纳米碳管生长模式研究

采用多元醇法制备镁-镍合金纳米粉末,并以此为催化剂制备纳米碳管,利用比表面和孔径分布测定仪、X射线衍射仪和透射电镜,研究镁-镍合金催化剂的性能和纳米碳管的生长模式。结果表明:Mg∶Ni值对镁-镍合金催化剂特性影响较大,其中Mg∶Ni为1的催化剂颗粒比表面积较大且平均粒径较小;聚乙烯吡咯烷酮(PVP)用量增大,有利于提高催化剂颗粒的比表面积、减小平均粒径,但用量过大不利于Mg2Ni合成。在以镁-镍合金为催化剂制备碳纳米管的过程中,首先在催化剂表面形成碳膜,随后形成的碳膜将前期形成的碳膜及催化剂颗粒向外推挤,催化剂颗粒移动后遗留下中空隧道,最终形成碳管,由于纳米碳管尖端的催化剂颗粒反应后失去催化活性,碳管的生长动力主要来自碳管根部。     

Effect of Co and Ni nanoparticles formation on carbon nanotubes growth via PECVD

The effect of cobalt (Co) and nickel (Ni) nanoparticle catalysts on the growth of carbon nanotubes (CNTs) were studied, where the CNTs were vertically grown by plasma enhanced chemical vapour deposition (PECVD) method. The growth conditions were fixed at a temperature of 700 °C with a pressure of 1000 mTorr for 40 minutes with various thicknesses of sputtered metal catalysts. Only multi-walled carbon nanotubes are present from the growth as large average diameter of outer tube (～10–30 nm) were measured for both of the catalysts used. Experimental results show that high density of CNTs was observed especially towards thicker catalysts layers where larger and thicker nanotubes were formed. The nucleation of the catalyst with various thicknesses was also studied as the absorption of the carbon feedstock is dependent on the initial size of the catalyst island. The average diameter of particle size increases from 4 to 10 nm for Co and Ni catalysts. A linear relationship is shown between the nanoparticle size and the diameter of tubes with catalyst thicknesses for both catalysts. The average growth rate of Co catalyst is about 1.5 times higher than Ni catalyst, which indicates that Co catalyst has a better role in growing CNTs with thinner catalyst layer. It is found that Co yields higher growth rate, bigger diameter of nanotube and thicker wall as compared to Ni catalyst. However, variation in Co and Ni catalysts thicknesses did not influence the quality of CNTs grown, as only minor variation in I_G/I_D ratio from Raman spectra analysis. The study reveals that the catalysts thickness strongly affects not only nanotube diameter and growth rate but also morphology of the nanoparticles formed during the process without influencing the quality of CNTs.     

Bulk preparation and characterization of mesoporous carbon nanotubes by catalytic decomposition of cyclohexane on sol–gel prepared Ni–Mo–Mg oxide catalyst

Multiwalled carbon nanotubes were synthesized using Ni–Mo–Mg oxide catalyst prepared by sol–gel technique. Carbon nanotubes were formed in situ by the reduction of nickel oxide (NiO) and molybdenum oxide (MoO₃) to Ni and Mo by a gas mixture of nitrogen, hydrogen and cyclohexane at 750 °C. Scanning Electron Microscopy (SEM) was used to confirm the formation of carbon nanotubes (CNTs). The pore size distribution of carbon nanotubes (CNTs) was investigated by N₂ adsorption and desorption. It was found that the pore size fell into the mesopore range: 2 < d < 50 nm. Interpretation was also made using Raman spectroscopy, Diffuse reflectance spectroscopy, X-ray diffraction and ESR spectra. This method is found to produce a very high yield weighing over 20 times of the catalyst. Based on the experimental conditions and results obtained a possible growth mechanism of the carbon nanotubes is proposed.     

Effects of hydrogen on the formation of aligned carbon nanotubes by chemical vapor deposition

Well-aligned carbon nanotubes with controllable properties were grown on porous silicon substrates by thermal chemical vapor deposition. The morphologies of the carbon nanotubes were varied with the introduction of H2 during the catalyst activation and/or carbon nanotube growth processes. It was found that H2 promotes the growth of carbon nanotubes while preventing the formation of spherical amorphous carbon particles. Without the introduction of H2 during the C2H2 thermal decomposition, aligned carbon nanotubes mixed with spherical carbon particles were formed on the substrate. However, with the introduction of H2, pure carbon nanotubes were synthesized. These nanotubes also had uniform diameters of 10-20 nm, which is much smaller than nanotubes synthesized without H2. The average growth rate of nanotubes was also affected by the introduction of hydrogen into the reaction chamber during nanotube growth. With the addition of hydrogen, the average growth rate changed from 78 nm/s to 145 nm/s. A possible growth mechanism, including the effect of a high ratio of H2 to C2H2, is suggested for the growth of these well-aligned carbon nanotubes with uniform diameters.     

Low temperature growth of carbon nanotubes in a magnetic field

We report on the growth of carbon nanotubes on a glass substrate at a low temperature of 450 °C by plasma-enhanced chemical vapor deposition in the presence of a magnetic field. The growth of carbon nanotubes can be realized at 450 °C only when a magnetic field is applied to the substrate. Carbon nanotubes cannot be grown in the absence of a magnetic field at the same temperature. An NH₃ plasma pretreatment significantly improved the uniformity of the grain size of the Ni catalyst under the magnetic field. The enhancement in the growth of CNTs at low temperature can be attributed to the magnetic moment pre-alignment of the ferromagnetic catalyst film under high magnetic field. A high emission current density of 20 mA/cm² was obtained at 6 V/μm and a stable emission current was observed. This method permits the growth of carbon nanotubes directly on glass substrate at much more reliable low temperatures for the fabrication of high-density field emitter arrays.     

Field emission study of carbon nanotubes forest and array grown on Si using Fe as catalyst deposited by electro-chemical method

Vertically aligned carbon nanotubes are synthesized by Low Pressure Chemical Vapor Deposition (LPCVD) on Si substrate coated with Fe as a catalyst at a pressure of 20 Torr and at a growth temperature of 600 degrees C. The catalyst film is prepared by electro-chemical method which is very unique and a low cost method. Three precursor gases Acetylene (C2H2), Ammonia (NH3) and Hydrogen (H2) at the flow rate of 20 sccm, 100 sccm and 100 sccm respectively are allowed to flow through the Low Pressure Chemical Vapor Deposition reactor for 10 minutes. Scanning Electron Microscope (SEM) images show that synthesized CNTs are vertically aligned and uniformly distributed with a high density. Raman analysis shows G-band at 1574 cm(-1) and D-band at 1370 cm(-1). The G-band is higher than D-band, which indicates that CNTs are highly graphitized. The field emission measurement reveals good field emission properties of as-grown vertically aligned carbon nanotubes with turn-on field of 1.91 V/microm at the current density 10 mA/cm2. The field enhancement factor is calculated to be 7.82 x 10(3) for as-grown carbon nanotubes.     

堇青石载镍催化剂对燃烧合成碳纳米管的影响

借助于硝酸镍溶液, 利用浸渍法在堇青石表面均匀负载镍催化剂颗粒, 在甲烷扩散火焰中活化并催化生成碳纳米 管. 实验结果表明, 生成的多壁碳纳米管直径为30～50nm, 长度约为十几微米, 空腔比较小, 管壁石墨结晶结构良好.提高浸渍液浓度, 催化剂颗粒尺寸明显变大, 但对碳纳米管的形态影响比较小. 延长浸渍时间, 可使催化剂颗粒密度提高, 碳纳米管出现成束生长现象. 结合碳管成核生长过程和火焰燃烧的特点, 探讨了催化剂对于碳纳米管生长的影响机制.     

Effect of Ethylene Flow Rate and CVD Process Time on Diameter Distribution of MWCNTs

To date, focus of the research activities in nanoscience was to control the chemical vapor deposition (CVD) growth of carbon nanotubes (CNTs) by changing the precursor pressure and process temperature. The effect of the precursor flow rate and process time on CNTs growth parameters has been overlooked in past studies and therefore is very little known. This study was focused on the optimization of the ethylene flow rate and CVD process time for CNTs growth over Fe₂O₃/Al₂O₃ catalyst in a fluidized bed chemical vapor deposition (FBCVD) reactor, operating at atmospheric pressure. Argon and hydrogen were considered as the carrier and supporting gases, respectively. Transmission electron microscope (TEM) and Scanning Electron Microscopy (SEM) were used to investigate the surface morphology, nanostructures, purity and yield of the grown CNTs. In-depth analysis revealed an increase in tube length, yield and the carbon concentration with ethylene flow rate in the range of 50–110 sccm. However, an inverse relationship between flow rate and tube diameter distribution was predicted in the given work. The most favorable results were obtained at an ethylene flow rate of 100 sccm and a CVD process time of 60 minutes. The dense and homogeneous growth of relatively pure nanotubes of increased tube length and narrow diameter distribution, in the range of 20–25 nm, was observed at optimized flow rate and process time.