Chapter 3 Results and Discussion
3.3 Sharp ZnO Nanorod Array
3.3.3 Etching of Nanorods by Plasma
3.3.3.2 The TEM Analysis of Structure
In order to make a detail investigation on structure changes of ZnO nanorods after plasma bombardment, we analyze the nanorods which are etched by Ar plasma for 60s with HRTEM and SAED. Figure 3-27 shows the TEM, HRTEM and SAED images of plasma-treated ZnO nanorod. As shown in this image, the nanorod top is sharp tip as the results of SEM. The inserts of this figure are the HRTEM images of column core and cone top, respectively.
These images illustrate that the ZnO wurtzite structure is complete without destruction. It suggests that there is no significant change on the surface crystal structure of plasma-treated nanorod. The other insert of this figure shows the SAED image of cone top. It indicates that the top of plasma-treated nanorod is still single crystal structure. It suggests again that the structure of nanorod is complete. Therefore, the field emission would be deteriorated through plasma bombardment due to complete ZnO nanorod structure. From this SAED image, it also illustrate that the growth direction of ZnO nanorod is along [0001].
3.3.3.3 The XRD Analysis of Structure
Figure 3-28 shows the XRD patterns of plasma-treated ZnO nanorod arrays with bombardment time of 120s. It can be observed that the (002) orientation of nanorods become weaker after etching. In addition, the FWHM of bombarded nanorods is 0.251° larger than as-grown (0.248°). It is indicated that the structure of ZnO nanorod array would be destroyed through ion
3.3.3.4 The Field Emission Measurement
The J-E curves of plasma-treated nanorods with different bombardment time are showed in Figure 3-29. The corresponding F-N plot is showed in Figure 3-30. The details of field emission properties with different bombardment time are showed in Table 3-4. As shown in this table, the nanorods which are bombarded for 60s exhibit the best field emission properties. It is because the tops of nanorods for 60s are sharper than others on the whole.
In this thesis, we choose the bombardment time of 60s as our etching parameters of plasma-treated ZnO nanorods for improving field emission property.
3.3.4 Two-Step Etching
ZnO nanorods have been etched by acid and plasma individually above for small radius of nanorod tip. To improve the field emission further, we combine the acidic etching and plasmatic etching to sharpen nanorod tops. At first, the nanorods are etched by acetic acid to form raised top. Different diameters of raised tops can be fabricated by different concentration and etching time. Then the raised tops are bombarded by Ar ion to form cone tips.
Different angles of cone tips can be fabricated by different bombardment time.
To fabricate sharper tip, the nanorods which grew in 0.0375M for 3hr at 85℃
are etched with concentration of 1:20000 and etching time of 15min at 85℃.
And then they are etched for different bombardment time with 15W under
3.3.4.1 The SEM Analysis of Morphology
Figure 3-31 is the SEM images of nanorods which are etched by two-step etching with different bombardment time. After acidic etching, the nanorod top is divided into two parts called raised top and remaining rod. For 5s, the raised top and remaining rod are still like as-etched top. When bombardment time is 10s, the raised tops become cone tips and the top edge of remaining rod is pared off. Up to 30s, the nanorod tops become sharp tip. The angle of tip is about 70° smaller than the angle with only plasmatic etching. It contributes to further improvement of field emission property. From 30s to 60s, the angles of nanorod tips are similar. About 120s, the nanorods become shorter and rounder.
It is like the plasmatic etching nanorods at 120s. It means that field emission property of nanorods would be deteriorated through long bombardment time.
Therefore, we must control the time of bombardment to fabricate the sharpest ZnO nanorod arrays which exhibit the best field emission property.
The reason that the nanorods with two-step etching are sharper than only plasmatic etching is as follows: the acidic etching can narrow the diameter of top to form raised top at first. The following ion bombardment would sharpen this raised top further. Since the diameter of raised top is smaller than as-grown nanorod tops, two-step etching tops would be sharper than plasmatic etching.
3.3.4.2 The Field Emission Measurement
The J-E curves of nanorods which are etched by two-step etching with
the nanorods which are bombarded for 30s exhibit the best field emission properties. It is because the tops of nanorods for 30s are sharper than others on the whole.
In this thesis, we choose 30s as our bombardment time in two-step etching for improving field emission property.
3.3.5 Field Emission Property Comparisons of
Sharp Nanorods
To apply ZnO nanorod array to field emission display, the turn-on electrical field (Eon) should be small and field enhancement factor (β) should be large. For this reason, we try four methods to sharpen ZnO nanorods. First, non-annealed nanorods are etched with a weaker acid (1:20000) for 15min at 85℃. Second, nanorods are annealed initially at 500℃ for 1min. Then they are etched with a stronger acid (1:5000) for 1hr at 85℃. Third, nanorods are etched by Ar plasma with 15W for 60s under 5x10-2 torr. Fourth, nanorods are etched initially in a weaker acid (1:20000) at 85℃ for 15min as first method. Then they are etched by Ar plasma with 15W for 30s under 5x10-2 torr. The J-E curves of sharpened ZnO nanorods by different methods are showed in Figure 3-34. The corresponding F-N plot is showed in Figure 3-35. The details of field emission properties with different methods are showed in Table 3-6. It shows that the field emission property of nanorods with raised top is better than as-grown. It is attributed to narrowing the radius of top. It also can be observed
destroying the structure. Compared to two-step etching, the angles of cone tops of plasma-treated nanorods are larger. It results in worse field emission property than two-step etching. Although the tops of annealed nanorods which are etched by acid are very sharp in these methods, the structures of them would be destroyed due to acidic etching. It contributes to even worse property than plasmatic etching.
Besides Eon and β, the stability of ZnO nanorod array is important to practical application of field emission display. To study the stability of nanorod arrays, the field emission property is measured through one thousand cycles.
And then the Eon values are extracted. Figure 3-36 is the Eon variations of as-grown, plasmatic etching, and two-step etching. From this figure, it is observed that the Eon of two-step etching is the most stable. It is because a nanorod with plat top generates current from different position every time. And a nanorod with sharp top generates current at its apex every time. Therefore, the stability of two-step etching with the sharpest top is the best. Form above results, it is indicated that the two-step etching exhibits not only the best field emission properties but also the best stability.
Table 3-1 The detail data of field emission properties of ZnO nanorods with different growth concentration
0.0125M 3hr 0.025M 3hr 0.0375M 3hr 0.05M 3hr Diameter 50~250 (nm) 300~400 (nm) 100~200 (nm) 200~300 (nm)
Length 1.0~1.5 (um) 2.0~2.3 (um) 1.2~1.4 (um) 0.8~1.0 (um) Eon 3.93 (V/um) 3.40 (V/um) 3.20 (V/um) 3.50 (V/um)
β 1476 1571 1647 1301
Table 3-2 The detail data of field emission properties of ZnO nanorods with different growth time
0.0375M 1hr 0.0375M 2hr 0.0375M 3hr 0.0375M 4hr Diameter 100~300 (nm) 100~200 (nm) 100~200 (nm) 100~200 (nm)
Length 0.6~0.8 (um) 0.9~1.1 (um) 1.2~1.4 (um) 1.2~1.3 (um) Eon 4.02 (V/um) 3.19 (V/um) 3.20 (V/um) 3.23 (V/um)
β 1414 1483 1647 1455
Table 3-3 The detail data of field emission properties of annealed ZnO nanorods which are etched with different acidic concentration
Eon (V/um) β
As grown 3.20 1647
1:20000 2hr 3.12 1718
1:10000 2hr 2.90 1765
1:5000 2hr 2.60 1975
Table 3-4 The detail data of field emission properties of plasma-treated ZnO nanorods for different bombardment time.
Eon (V/um) β
As grown 3.20 1647
30s 2.51 2546 60s 2.26 2696 120s 3.07 1614
Table 3-5 The detail data of field emission properties of ZnO nanorods which are sharpened by two-step for different bombardment time.
Eon (V/um) β
As grown 3.20 1647
30s 1.44 3415 60s 1.81 3375 120s 3.00 1782
Table 3-6 The detail data of field emission properties of sharp ZnO nanorods by different method.
Eon (V/um) β
As grown 3.20 1647
Etching non-annealed nanorods 2.85 1972
Etching annealed nanorods 2.60 1975
Plasmatic etching 2.26 2696
Figure 3-1 The XRD patterns of ZnO seed layer which is annealed at different temperature.
Figure 3-3 The SEM images of annealed ZnO seed layer on Si substrate. (a) Top and (b) cross view
(a)
(b)
Figure 3-4 The optical transmittance of annealed ZnO seed layer
(a) (b)
(c) (d)
(e) (f)
(g) (h)
(a) (b)
(c) (d)
(e) (f)
(g) (h)
Figure 3-8 The XRD patterns of ZnO nanorods which are annealed for different time
Figure 3-10 The bended band diagram of ZnO nanorod (a) before annealing (b) after annealing in oxygen
Ec
EF
Ec
Ev Ev
boundary
depletion
(a) (b)
Figure 3-12 The field emission F-N plots of ZnO nanorods with different growth concentration
Figure 3-14 The field emission F-N plots of ZnO nanorods with different growth time
Figure 3-15 The SEM images of non-annealed nanorods which are etched with 1:20000 concentrations for different etching time. (a) 5min (b) 15min (c) 1hr (d) 2hr (e) 4hr (f) 5hr.
(a)
(e) (f)
(c) (d)
(b)
Figure 3-16 The etching mechanism of non-annealed nanorods
Figure 3-17 The SEM images of non-annealed nanorods which are etched with
(a) (b)
(c) (d)
Defect
OH－OH－
OH－ OH－ OH－OH－
Figure 3-18 (a)(c)(e) are tilted views of nanorods which are etched with 1:10000 concentration for 1hr, 2hr, and 4hr after short time annealing, respectively. And (b)(d)(f) are corresponding cross views.
(a) (b)
(c) (d)
(e) (f)
Figure 3-19 The etching mechanism of annealed nanorods Defect
Etching
Annealing Etching
OH－OH－ OH－ OH－
Figure 3-20 (a)(c)(e) are tilted views of nanorods which are etched with 1:20000, 1:10000, and 1:5000 concentration for 2hr after short time annealing, respectively. And (b)(d)(f) are corresponding cross views.
(a) (b)
(c) (d)
(e) (f)
Figure 3-21 (a)(c)(e) are tilted views of nanorods which are etched for 2hr with 1:20000, 1:10000, and 1:5000 concentration after long time annealing, respectively. And (b)(d)(f) are corresponding cross views.
(a)
(c) (d)
(e) (f)
Figure 3-22 The XRD patterns of annealed nanorods which are etched with different acidic concentration
Figure 3-24 The field emission J-E curves of annealed ZnO nanorods which are etched with different acidic concentration
Figure 3-26 (a)~(e) are the SEM image of plasma-treated nanorods for bombardment time of 10s, 20s, 30s, 60s, and 120s, respectively.
(a) (b)
(c) (d)
(e)
Figure 3-27 The TEM and SAED image of plasma-treated nanorods for bombardment time of 60s.
Figure 3-28 The XRD patterns of plasma-treated nanorods which are bombarded for 120s
Figure 3-30 The field emission F-N plots of plasma-treated ZnO nanorods for different bombardment time
(a) (b)
(c)
(e)
(d)
(f)
(g)
Figure 3-32 The field emission J-E curves of ZnO nanorods which are sharpened by two-step etching for different bombardment time
Figure 3-34 The field emission J-E curves of sharp nanorods by different methods
Figure 3-36 The stability measurement of sharp nanorods by different methods
Chapter 4 Conclusion
We choose the growth concentration of 0.0375M and growth time of 3hr as growth parameters since the field emission properties of nanorod array and the uniform level of nanorod distribution are both best under these condition.
The Eon and β of this nanorod array are 3.20 (V/um) and 1647, respectively.
To improve field emission properties, we advance four methods to sharpen ZnO nanorods. First, non-annealed nanorods are etched with a weaker acid (1:20000) for 15min at 85℃. The flat tops of nanorods would become raised. The Eon and β of this sharp nanorod array are 2.85 (V/um) and 1972, respectively. Second, nanorods are annealed initially at 500℃ for 1min. Then they are etched with a stronger acid (1:5000) for 1hr at 85℃. The Eon and β of this sharp nanorod array are 2.60 (V/um) and 1975, respectively. Third, nanorods are etched by Ar plasma with 15W for 60s under 5x10-2 torr. The Eon
and β of this sharp nanorod array are 2.26 (V/um) and 2696, respectively.
Fourth, nanorods are etched initially in a weaker acid (1:20000) at 85℃ for 15min as first method. Then they are etched by Ar plasma with 15W for 30s under 5x10-2 torr. The Eon and β of this sharp nanorod array are 1.44 (V/um) and 3415, respectively. From above results, two-step etching (fourth method) exhibits the best field emission properties. It can reduce the power consumption of field emission display effectively. In addition, two-step etching is also the most stable in these methods.
Besides field emission property, we also illustrate the etching process of
sharp morphologies exhibit higher surface energy so that they are easily etched by acid. The etching process starts here to form raised tops. And then concave tops are formed by selective etching of ZnO nanorod along the C-axis because the (0001) face exhibits high surface energy and there are many defects near the center of nanorod. For etching of annealed nanorods, the annealing process enhances structures of nanorods. And the defect become less and uniform so that the etching process becomes isotropic from the corners to center.
In this thesis, we fabricate a ZnO nanorods array with sharp tops to improve field emission properties. Besides sharpening nanorods, decrease of nanorod density also can enhance the properties due to diminishing screen effect. Therefore, the study on synthesis of ZnO nanorod array with low density for better field emission properties is our future work.
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