Thin films of functionalized single-wall carbon nanotubes were deposited on silicon chips by drop-coating and inkjet printing. These sensors were subjected to 1-100 ppm NOx, CO, H2S and H2O vapor in synthetic air. We have found that besides the expected changes in the electrical resistance of the film, there are also characterteristic differences in the noise pattern of the resistance vs. time function. This phenomenon is called fluctuation enhanced sensing and it can be used to increase the amount of information gathered from a carbon nanotube sensor device. The main advantage of fluctuation enhanced sensing is the improved selectivity of the sensor even if changes in electical resistance are rather low. Combined with differentiation based on modifying the adsorption characterstics of the nanotubes (e.g. by covalent functionalization), fluctuation enhanced sensing appears to be a very useful method for bringing cheap and reliable carbon nanotube based chemical sensors to the market.
The functionalization of carbon nanotubes (CNTs) is important both for composite - to improve load transfer between CNTs and matrix - and nanoelectronic applications - to interlink individual nanotubes in a network. Oposite to earlier results, complete coverage of CNT surface with functional groups was achieved. The distribution of functional groups on the nanotube surface was investigated using STM and TEM. The influence of functional groups on the electron density of states of the nanotubes was studied with scanning tunneling spectroscopy (STS).
A quite wide brunch of the carbon nanotube science, including the utilization of singlewall nanotube for production of nano-electronic devices has being continuously explored even nowadays. Tuning and modifying the synthesis procedures to obtain nanotube junctions of T, Y, H or X shapes lead to inappropriate results concerning the industrial or large scale production. However, the importance and the demand for these junctions are quite large, since these may be the secondary building units of carbon nanotubes based chips or even more complex nanoelectronic devices. Recently, some novel solutions of their preparation have been published. A Taiwanese group described a method to prepare multi-junctioned carbon nanotubes on mechanically pretreated silicon surface applying chemical vapor deposition (CVD) technology using decomposition of methane at 1373 K. The nanotubes were nucleated following the lines prepared by scratching the surface with 600-grit sand paper. Contrary to the physical pretreatment of a substrate surface, chemical reactions can also be used for the preparation of carbon nanotube junctions. P.W. Chu et al. reported interconnecting reactions between functionalized carbon nanotubes . By the described method, the carboxyl groups on the wall of singlewall carbon nanotubes are converted to carbonyl chloride groups by reaction with SOCl2 at room temperature. The formed COCl groups are very reactive on the outer surface and can be reacted easily with various amines, particularly diamines resulting in the formation of amide bonding. When two functionalized carbon nanotubes react with such an amine molecule interconnection of tubes is generated. The resulted carbon nanotube junctions have been investigated by AFM.
In this presentation, we report on the results obtained on the preparation of carbon nanotube junctions applying two different procedures. The first method is similar to Chu’s one, which was mentioned above, i.e. we used functionalized multiwall carbon nanotubes and the successful interconnection of them by propylene diamine has been proven by TEM and AFM. The second method demonstrates a novel principle: catalyst material has been deposited on the outer surface of carbon nanotubes and branches of nanotubes were produced at this contact point by catalytic chemical vapor deposition (CCVD) of acetylene. The product has been characterized by TEM.
Investigation of the properties of nanometer sized particles is in the focus of material and chemical science. Several questions about the synthesis and application of carbon nanotubes have been addressed the researchers working in nanoscience and nanotechnology. The catalytic chemical vapor deposition (CCVD) method proved to be one of the most prosperous technologies for large scale production of both single and multiwall carbon nanotubes. It has been proven that supported transition metals are the most productive catalysts for CCVD. The bimetallic catalysts showed excellent catalytic activity in conversion of acetylene to multi wall carbon nanotubes (MWNT) .
In our previous papers, we dealt with the cobalt-iron bimetallic catalytic system. We showed that the best quality MWNTs with high yield was observed on Co-Fe catalyst. From the in situ XPS results we concluded that Co-Fe alloy phase should be formed on the catalyst treated at high temperature in acetylene atmosphere, furthermore, we attributed the effectiveness of this catalyst to the presence of alloy phase. However, we have also indicated the importance of Mössbauer spectroscopic studies. In this contribution we report on the results of Mössbauer spectroscopy supplementing our previous conclusions drawn by chemical and XPS techniques.