of the cone with growth the diameter of the cone decreases with growth with nitrogen doses beyond 20 sccm fig1 illustrates the control over the conical angles with different nitrogen doses by appropriately timing the oxygen dosing during the growth we could successfully control the internal diameters of the carbon tubes as mentioned earlier using oxygen dosing during the second half of the growth we could synthesize u2018 tube on cone u2019 morphologies by dosing oxygen 0 10 min 20 min and 30 min after the start of the experiment we could increase the diameters of the carbon tubes from 150 nm to about 1 5 microns the diameter distribution of the carbon tubes obtained in these experiments is shown in fig1 and fig1 the total time for each experiment lasted one hour the principle involved in this method is that as the cone is growing the diameter of the cone increases with time therefore by appropriately introducing oxygen at different times during the growth we can tune the diameter of the tube that grows after the oxygen dosing the diameter of such tubes can be increased to values as high as 25 microns provided there is enough gallium to sustain the growth in our experiments the amount of gallium required for such large diameters is limited due to the use of thin film of gallium this limitation can however be overcome by constantly supplying gallium through the gas phase by using gallium precursors such as tri methyl gallium the carbon wall.