Carbon Nano tubes as field emitter tip
Last few decades CNTs (Carbon Nano Tubes) have drawn our attention because of its wide application. CNTs are molecular- scale tubes of graphite carbon with outstanding properties. They are among the stiffest & strongest fibre known. They have a remarkable electronic property and many other unique characteristic. One of the potential applications of CNTs is field emitter tip. The advantage of using CNTs as field emitter is because of its small radius of curvature, high aspect ratio, high chemical inertness & mechanical strength.
The main purpose of this research is to fabricate CNTs for electron field emission. We even concern about the low turn- on field & high field enhancement factors. There are many methods/process for the fabrication technique of CNTs. The two concepts in CNTs are explained in terms of low filed screening effect.
Two methodologies suggest for the preparation/fabrication of CNTs are:
- Using anodizing process followed by pyrolysis of C2H2 at high temperature.
- Photolithography process.
In anodizing process we fabricate anodic aluminium oxide (AAO) by anodizing Al film. Here we take two types of aluminium sample. One of 12 µm- thick sputtered Al on silicon wafer (AL) & another sputtered Al film on Nb-coated silicon wafer (ALNB). We take two samples to study the current-voltage relationship & also the field enhancement factor of the samples in order to reduce the screening effect. The AL sample is electro polished in a mixture of perchloric acid/ethanol solution to obtain mirror finish where as the ALNB already obtain it's smooth surface because it contains 2at % of Nd for the smooth surface. Here they take Co as catalyst from 5% CoSO4.7H2O solution stabilized by 2% H3BO3 by electrochemical deposition. They have applied a voltage of 15 Vrms for 1min & 3Vrms for 10s for the AL and ALNB sample. Co particle were reduced in gas mixture of 2% H2 & (8% Ar at 600c for 1 hr. Then the CNTs are grown by catalytic pyrolysis at 10%C2H2 & 20% H2 in an Ar carrier gas for 15min at 650C in tube reactor. The result shows a high enhancement value and low turn on field which results in low density of CNTs. To avoid screening effect a low density of CNTs are favourable.
In the photolithography process we fabricate carbon nanofiber/ nanotube patterns on substrate on large scale depending on the temperature of pyrolysis. In this process we directly photolithographic the pattern of metal-containing photoresist onto the substrates. The photoresist consists of poly (vinyl alcohol) (PVA), N-methyl-olacrylamide (cross linked agent), triphenylsulfonium salt (acid/free-radical generator) and metal salts (Ni (NO3)2, Fe (NO3)3 or Co (NO3)2). Then over photoresist a film of black and white with feature size 20µm is used as photomask. Then we remove the polymer matrix and reduce the metal oxide to metal nanoparticle using H2. Then CNTs are grown by the catalytic pyrolysis of hydrocarbon like acetylene.
In both the process we are fabricating the CNTs using pyrolysis of hydrocarbons like acetylene. Both the processes are time oriented and are temperature dependence. In the anodizing process we sputter an film of Al on a silicon wafer and then we use a catalyst for the reduction process, where as in the photolithography process we directly photolithograph the metal contain photoresist on to the substrate. Then we do the pyrolysis process for the growth for CNTs in both the process. At different temperature as mentioned in the methodologies above. It is found that if the pyrolysis temperature is above 7500C CNTs are generally formed and 6500C for nanofibers. The Nanotubes formed are seen by the TEM whereas the field emission was observed by the SEM.
Though the process of anodizing shows much better result than the photolithography process we want an industrial oriented process which can produce the output on large scale. Hence we go for the photolithography process which is more feasible, inexpensive process compare to anodizing process. In the anodizing process it's at time oriented process and quantity of sample produce are less where as in the lithography process the sample produce are more and less time orientation.
There are many work produce recently about fabrication of CNTs like
- Z.F.Ren developed a technique to grow vertically aligned carbon nanofibers at low temperature (6600) 
- Dai developed the patterned growth of singled walled carbon nanotubes on SiO2/Si by using photolithography process.
- Vertical aligned carbon nanotubes using plasma enhanced chemical vapour deposition.
- Nucleation & growth of carbon nanotubes by microwave plasma chemical vapour deposition.
Some of the future techniques have been developed for the growing the 2D & 3D CNTs includes physical masking, photolithography and soft lithography based on pyrolysis of FePc.
Applications of developed CNTs:
- Flat Panel display
- Nano electronics
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