An/Ni-catalyzed Growth And Field Emission Properties Of Carbon Nanotubes

In recent years, many one-dimensional nanostructured materials were preparedwith the rapid development of nano-technology, and in the preparation process, thecatalyst plays an important role. As a representative one-dimensionalnanostructured material, CNTs possess many unique and excellent properties,especially the field emission properties. In order to improve the field emissionproperties of the CNTs, synthesizing high-quality CNTs is necessary. In our work,a new catalyst, Au/Ni composite film, was employed to synthesize verticallyaligned CNTs by plasma enhanced hot filament CVD method in the ambient of H2and CH4.In the preparation process, two different methods, NORMAL method (withoutmask), and MASK method, were used. Firstly, with the growth parameters (i.e. thetemperature, the pressure, the bias current, and the blending ratio of the gasmixture) being changed, the growth characteristics of CNTs deposited byNORMAL method were studied. The optimized growth parameters are thetemperature of 800℃, the pressure of 8KPa, the bias current of 180mA, and theCH4/H2 ratio of 1:5. Under such conditions, the diameter of aligned CNTs is in therange of 20nm-30nm. Then, by MASK method, a mixture of silicon tips andvertical aligned carbon nanotubes was synthesized at high concentration ofhydrogen (96%) and a relatively high total pressure of 16.7 KPa. The diameter ofCNTs in the mixture is less than 10nm, and CNTs have clear and straight walls.With the thickness ratio of Au/Ni film varied, the shape and structure of CNTschange evidently, which indicates that the thickness ratio of Au/Ni film largelyaffects the growth characteristics of CNTs.The growth mechanism of CNTs deposited on Au/Ni film was studied. Bycomparing the formation of Au/Ni alloy, and the distortion of the catalyst film, andby calculating the activity of carbon in the alloy, we deduce that the addition of Aupromotes the absorption, diffusion, and separation process of carbon in the catalyst,and make it more easily saturated in the catalyst, and consequently promotes thegrowth of CNTs, so that CNTs can be prepared with high H2 concentration. Byfurther calculation, we find the law of the influence of Au-Ni thickness ratio on thecarbon activity. Different Au concentration leads to a different carbon, and Auactivity, and thereby leads to a different growth rate, morphology, and structure ofCNTs on Au/Ni film.Field emission properties of CNTs deposited with different growthparameters were studied. The results show that CNTs prepared in the experimentsexhibits good field emission properties. The mixture of silicon tips and CNTs has athreshold field of only 0.47V/μm, and its electric field becomes 1.48 V/μm withthe emission current density reaching 1mA/cm2. Au/Ni film with different Au-Nithickness ratio was employed as catalyst, and CNTs deposited on Au/Ni films of40nm/40nm, and 40nm/20nm has better field emission properties, which isprobably due to not only the morphology, but also the defects of CNTs. We studiedthe factors influencing the field emission properties of CNTs. Besides themorphology, and the structure of CNTs, the ambient of high H2 concentration, andthe use of Au/Ni film may largely affect the field emission properties of CNTs.The reason is probably due to that high concentration of H2 may increase the C-Hbond formed on the surface of CNTs, and decrease its surface potential barrier, andconsequently be favorable to the emission of electrons from the surface. The use ofAu/Ni film enables preparing thinner CNTs, and the residual Au particles on thesubstrate may help to transfer electrons to the emission positions. The presence ofsilicon tips in the mixture make the density of CNTs more appropriate for a betterfield emission property. CNTs was annealed at vacuum, H2 ambient, and N2ambient, respectively. The results show that the substrate temperature of 600℃may improve the field emission properties of CNTs annealed at vacuum, while900℃ is bad for its field emission properties. And for CNTs annealed at H2 or N2ambient, H2 is better to its field emission properties than N2.The growth characteristics, and the field emission properties of CNTs dopedwith nitrogen were studied. Through SEM, TEM, and Raman spectra, we knowthat the morphology, and the structure of CNTs deposited with different nitrogenflow rates change evidently. ID/IG ratio at Raman spectra, and the changes of C1sspectra of XPS reveal that the defects in CNTs first increase and then decreasewith increasing nitrogen flow rate, while nitrogen concentration increases withincreasing nitrogen flow rate. Au/Ni film thickness ratio, also, largely affects themorphology, and the structure of CNTs deposited with nitrogen flow rate keptconstant. Nitrogen concentration in nitrogen-doped CNTs was determined by XPS.With Au film thickness kept constant, nitrogen flow rate first increases sharply andthen decreases slowly with increasing Ni film thickness. With Ni film thicknesskept constant, nitrogen concentration first decreases and then increases. Fieldemission measurements on CNTs deposited with different nitrogen flow rates werecarried out, and the results show that field emission properties of nitrogen-dopedCNTs are better than that of undoped CNTs, and they become better withincreasing nitrogen flow rate. With nitrogen flow rate kept constant, the fieldemission properties of CNTs deposited on different substrates (i.e. the substrateswith different Au/Ni film thickness ratio on it) make a great difference, and theadoption of the substrates with 40nm/20nm, and 40nm/40nm Au/Ni film leads to abetter field emission property, which is probably owing to the influence of Auconcentration on the absorption of nitrogen.