Fig. 4.4(a)-(d) display SEM images of cylindrical ZnO NWs with increasing average diameters. The cylindrical NWs with average diameters of 12, 19, 38, and 113 nm were grown by using gold-nanoparticle catalysts with average sizes of 10, 20, 40, and 100 nm, respectively. The NWs displaying in the same magnification SEM images demonstrate obviously distinct dimensions, implying a very well control of the NW diameter through the size of gold nanoparticles. All these ZnO NWs were synthesized with the same conditions of temperature and time. We found the diameter of the NWs were just controlled by and linearly depended on the size of gold catalysts. From the SEM images, we can also observe that the thinner NWs (Fig. 4.4(a) and (b)) tempted to exhibit a more bended shape in contrast to the thicker NWs being straight. Fig.
4.4(e)-(h) displays statistical distribution, which was examined from TEM images, of our ZnO NW diameters. The average diameters with statistical distributions are 12±2.7, 19±3.2, 38±4.6 and 113±20 nm for the various ZnO nanowires, corresponding to the standard deviations of 23%, 17%, 12% and 18%, respectively. This somewhat large deviation in NW diameter may come from a broad size distribution of the gold catalysts.
A high-resolution TEM image of an as-grown ZnO nanowire with 10-nm diameters is shown in Fig. 4.5(a). Evident atomic columns reveal single crystalline structure and the [0001] growth direction. The periodic lattice structure indicates a double layer spacing
Figure 4.4: FESEM images of as-grown ZnO NWs with average diameters of (a) 12 nm, (b) 19 nm, (c) 38 nm, and (d) 113 nm. The corresponding statistical distributions
of 0.52 nm, in accordance with the distance between the (001) planes of the ZnO wurtzite structure. The inset of Fig. 4.5(a) shows the nano beam electron diffraction pattern of the 10-nm diameter ZnO nanowire with zone axis of [1120]. The electron diffraction pattern reveals fine dots, suggesting perfect lattice order free from those structure defects such as the orientational variations found in our previous work [49]. As-grown and pure ZnO NWs were doped by high energy Co ions to form Zn1-xCoxO NWs. A TEM image of an as-implanted Zn1-xCoxO NWs with an average diameter of 38 nm and a Co ion dose of 6 × 1016 cm-2 are demonstrated in Fig. 4.5(b). The as-implanted Zn1-xCoxO NWs consist of lots of stacking faults, as designated by triangles in Fig. 4.5(b), and they exhibit a streaking of an electron diffraction pattern (see the inset). Although the bending feature can be detected in as-grown ZnO NWs, there are more stacking faults and an obvious streaking in an electron diffraction pattern discovered in as-implanted Zn1-xCoxO NWs. It is proposed that these structure defects could mainly come from a high-energy Co ion bombardment during the ion implantation process. After a high-vacuum annealing, a TEM image of the same sample are shown in Fig. 4.5(c).
We can see that the stacking faults, which were indicated as triangles, and streaking are removed after a thermal treatment. We notice that an annealing in a high vacuum at 600°C could help to recover structure disorders and defects. We also found that annealing at a higher temperature may cause a meltdown of ZnO NWs. A lower annealing temperature of 450°C was thereafter applied to subsequent experiments and no noticeable changes in morphology were observed after the thermal treatment.
In order to study the magnetic mechanism in Zn1-xCoxO NWs, we have made a comparative sample, ZnO NWs sheathed in amorphous carbon by Co ion implantation.
There are neither perceptible clusters nor nanocrystals before any thermal treatments.
After a high-vacuum annealing, Co clusters, having a broad size distribution, could be
Figure 4.5: (a) High resolution TEM image of an as-grown ZnO NW with the diameter of 10 nm. The inset shows its corresponding nano beam electron diffraction pattern. High resolution TEM images of (b) as-implanted and (c) high-vacuum annealed Zn1-xCoxO NWs. The inset in (b) and (c) displays corresponding electron diffraction patterns. (d) TEM image of a ZnO NW sheathed in carbon amorphous with Co clusters after high-vacuum annealing. The inset of (d) is the statistical distribution of Co-cluster diameters estimated from TEM images. The average diameter and
discovered in TEM images. Fig. 4.5(d) shows a typical TEM image of the Co clusters with a statistical distribution of diameters in the inset. The 40-nm diameter ZnO NW is embedded in a shell of carbon amorphous with a diameter of ~100 nm. The average diameter and standard deviation of the Co clusters are about 9.4 and 6.0 nm, respectively. This sample was fabricated by Co ion implantation with a dose of 4 × 1016 cm-2 and post-annealed in a high vacuum at 600°C. In contrast to the high-vacuum annealed Zn1-xCoxO NWs, in which Co cluster have never been detected in TEM images (Fig. 4.5(c)), the sample of ZnO sheathed in amorphous carbon with Co-ion implantation exhibits obviously many Co clusters after a high-vacuum annealing. The result suggests that Co ions may have a longer diffusion length in amorphous carbon than that in ZnO.
The Co concentration of the as-implanted Zn1−xCoxO nanowires was checked by using EDX. The atomic ratio between Co and Zn was used to determine the Co concentration of this particular nanowire being 10.9%. The average Co concentrations of our as-implanted nanowires are 4, 8 and 11% for the Co ion doses of 2, 4 and 6×1016 per cm2, respectively. Noticeably, our EDX analysis indicates that the average value of Co concentration does not show apparent variations on nanowire diameters, once the same dose was used for implantation. In addition, we have carried out EELS mapping studies, shown in Fig. 4.6, to further confirm a homogeneous distribution of Co element in our Zn0.92Co0.08O nanowires after annealing in a high vacuum. Fig. 4.6 also shows images of the O K and Zn L edges to corroborate the compositional mapping of EELS. The image of Co L edge sustains that no Co clusters exist in our Zn0.92Co0.08O nanowires.
Therefore, we can safely conclude that our Zn0.92Co0.08O nanowires might be intrinsic DMS materials.
Figure 4.6: Bright-field image with compositional EELS mapping of the Co L, O K, and Zn L edges of the high-vacuum annealed Zn0.92Co0.08O nanowires with an average diameter of 38 nm.