Chapter 2: Experiment
3.2 Finding Optimum R/H Ratio of Different Pillar Spacing
3.2.1 Effect of carbon source flow rates on the morphology of CNTs’ growth · 48
growth
It is indispensable to reduce the turn on field and threshold field to achieve practically applicable field electron emitters that operate at lower power consumption. It has been reported that field emission can effectively enhanced for aligned CNTs as field emitters when the ratio of distance between neighboring nanotubes to the height of each individual CNT is about 2 [3.4]. However, the optimum rule (R/H=2) for CNTs as electron field emitters has not been realized.
In our researches, different R/H ratios have been designed to study the relation between R/H ratios and the field emission characteristics, and it is essential to grow longer aligned CNTs’ pillar with larger inter- pillar distance to meet the R/H ratios we need. However, growth of longer CNTs beyond 100μm is a little bit difficult. Therefore, it is very important to study the relation between the length of CNTs′ pillar and the growth condition including growth time, flow rate of carbon source, etc.
The length of the aligned CNTs pillar as a function of growth time is shown in Fig 3.2.4(a). It is observed that there is a CNTs′ length limit of approximately to 130μm. In
this case of the aligned CNTs′ pillars grown on the aluminum substrate, the aligned CNTs can′t be lengthened even if prolongs the growth time beyond 90 min. When we increase the flow rate of C2H4 to 135sccm, its growth rate is about 2μ m/min faster than that at 60sccm at 700oC in growth process. But the length of CNTs′ pillar does not match our prediction. Fig 3.2.4 (b) shows the longest CNTs′ pillar is about 50μ m , even if prolongs the growth time beyond 40min. It is probably due to too much amorphous carbon putting on catalytic nanoparticles. Too much carbon source to close crystalline graphite sheets of the wall and encapsulate nanoparticle at the closed tip is another probable reason.
According to this result, we try to reduce the flow rate of C2H4 from 135sccm to 60sccm.
Thus, moderate carbon source flow rate is indispensible to grow longer CNTs′ pillar.
Besides, the tendency that CNTs′ growth rate is linear with the time until the length limit is reached is also observed from Fig 3.2.4. Therefore, moderate flow rate of carbon source as well as growth time is required to precisely control once we want to grow the specified length of CNTs′ pillar. According to our experimental results, we can easily grow longer length of CNTs′ pillar to meet the required R/H ratios we need.
3.2.2 Effect of arrangement of pillar array on its field-emission characteristic
Three circle patterns with different inter-pillar distance (80μ m,150μ m and 250μ m ) are designed to get different R/H ratios. In the previous section, we have succeeded to find the suitable growth condition to grow longer length of CNTs′ pillar to meet the required R/H ratios we need.
The aligned CNTs pillar is characterized by high-resolution scanning electron microscopy (SEM). Aligned CNTs pillar that is perpendicular to the substrate surface is obtained due to J.van der Waals force between CNTs. Fig.3.2.1 ~ Fig.3.2.3 show SEM images of an aligned CNTs pillar grown on silicon substrate with aluminum buffer layer.
(a) 80μm inter-pillar distance case
Fig.3.2.1 (a) shows the aligned CNTs bundle with a diameter of 50μ m and a height of 4.5μ m. The R/H value of 18.18 is obtained. Fig.3.2.1 (b) shows the aligned CNTs bundle with a diameter of 50μ m and a height of 8μ m. The R/H value of 11.2 is
obtained. Fig.3.2.1 (c) shows the aligned CNTs bundle with a diameter of 50μ m and a height of 10μ m. The R/H value of 8 is obtained. Fig.3.2.1 (d) shows the aligned CNTs bundle with a diameter of 50μ m and a height of 10.8μ m. The R/H value of 7.8 is obtained. Fig.3.2.1 (e) shows the aligned CNTs bundle with a diameter of 50μ m and a height of 13.2μ m. The R/H value of 6.06 is obtained. Fig.3.2.2 (f) shows the aligned CNTs bundle with a diameter of 50μ m and a height of 22.3μ m. The R/H value of 3.6 is obtained. Fig.3.2.1 (g) shows the aligned CNTs bundle with a diameter of 50μ m and a height of 53μ m. The R/H value of 1.5 is obtained.
( b ) For 150μm inter-pillar distance case
Fig.3.2.2 (a) shows the aligned CNTs bundle with a diameter of 50μm and a height of 14μm. The R/H value of 10.7 is obtained. Fig.3.2.2 (b) shows the aligned CNTs bundle with a diameter of 50μm and a height of 20μm. The R/H value of 7.5 is obtained.
Fig.3.2.2 (c) shows the aligned CNTs bundle with a diameter of 50μm and a height of 26μm. The R/H value of 6 is obtained. Fig.3.2.2 (d) shows the aligned CNTs bundle with a diameter of 32μm and a height of 14μm. The R/H value of 4.7 is obtained. Fig.3.2.2 (e) shows the aligned CNTs bundle with a diameter of 50μm and a height of 47μm. The R/H value of 3.2 is obtained. Fig.3.2.2 (f) shows the aligned CNTs bundle with a diameter of 50μm and a height of 60μm. The R/H value of 2.5 is obtained. Fig.3.2.2 (g) shows the aligned CNTs bundle with a diameter of 50μm and a height of 82μm. The R/H value of 1.87 is obtained.
( c ) For 250μm inter-pillar distance case
Fig.3.2.3 (a) shows the aligned CNTs bundle with a diameter of 50μm and a height of 17.9μm. The R/H value of 14 is obtained. Fig.3.2.3 (b) shows the aligned CNTs bundle with a diameter of 50μm and a height of 23μm. The R/H value of 10 is obtained.
Fig.3.2.3 (c) shows the aligned CNTs bundle with a diameter of 50μm and a height of 49μm. The R/H value of 5.1 is obtained. Fig.3.2.3 (d) shows the aligned CNTs bundle with a diameter of 50μm and a height of 80μm. The R/H value of 3.1 is obtained.
Fig.3.2.3 (e) shows the aligned CNTs bundle with a diameter of 50μm and a height of 100μm. The R/H value of 2.5 is obtained.
From the SEM images in the above section, patterned CNT pillars with different
R/H ratios we need have been fabricated successfully using thermal chemical vapor deposition (CVD) method. Therefore, it is easily to study the effect of R/H ratios on the field emission characteristics of CNTs′ pillar arrays.
3.2.3 Effect of different CNTs′ pillar arrays with different R/H ratios on the field-emission characteristic
In our experiment, we get high density, vertical, long length and aligned CNTs pillar structure with different inter-pillar distance. Then, the relation between R/H ratios and field-emission characteristics from the CNT pillar array are studied. Fig.3.2.5(a) shows field emission current density (J) as a function of applied electric field (E) for CNT pillar arrays with heights of 4.5μm, 8μm, 10μm, 10.8μm, 13.2μm, 22.3μm and 53μm (R/H=18.18, 11.2, 8, 7.8, 6.06, 3.6,and 1.5,respectively). We find that the best field-emission characteristic is obtained from the pillar array with R/H=8 (H=10μm) and it will be discussed later. Fig3.2.5(c) shows the Fowler–Nordheim (FN) plots corresponding to Fig3.2.5(a) .The occurrence of field emission is confirmed by the linearity of the FN plot. Fig 3.2.6(a) shows the turn-on field (Eturn-on) obtained from Fig.
3.2.5(a) as a function of R/H. Eturn-on is defined to be the field required to produce J=10 μA/cm2.The optimal value of R/H giving a value of minimum Eturn-on is approximately 8.
which corresponds to H=10μm. When R/H is smaller than 8, the field-emission characteristic is poor, which is likely to be due to the field screening effect. However, when the R/H is greater than 8, Eturn-on increases, and the reduction of field enhancement owing to a small H is conjectured.Fig3.2.6(b) shows R/H versus the field enhancement factor (β), [3.12] which is estimated from the slope of the FN plots in Fig. β is estimated experimentally by setting the work function of the CNTs to be 5 eV. The obtained values of β for the CNT pillar array with R/H values of 8, 11.2, 6.06 and 1.5 are 61060, 54540, 29075 and33031, respectively. The best field enhancement factor is obtained at R/H~8.
In the case for inter-pillar distance of 150μm, the relation between R/H ratios and field-emission characteristics from the CNT pillar array are studied. Fig3.2.7 shows emission current density (J) as a function of applied electric field (E) for CNT pillar arrays with heights of 14μm, 20μm, 26μm, 32μm, 47μm, 60μm and 82μm (R/H=10.7,7.5,6, 4.7,3.2,2.5and1.87,respectively). We also find that the best
field-emission characteristic is obtained from the pillar array with R/H=4.7 (H=32μm), and it will be discussed later. Fig.3.2.7(c) shows the Fowler–Nordheim (FN) plots corresponding to Fig3.2.7(a). The occurrence of field emission is confirmed by the linearity of the FN plot. Fig.3.2.8(a) shows the turn-on field obtained from Fig.3.2.7(b) as a function of R/H. The optimal value of R/H giving a value of minimum Eturn-on is approximately 4.7, which corresponds to H=32μm. When R/H is smaller than 4.7, the field-emission characteristic is poor, which is likely to be due to the field screening effect.
However, when the R/H is greater then 4.7, Eturn-on increases, and the reduction of field enhancement owing to a small H is conjectured.Fig.3.2.8(b) shows R/H versus the field enhancement factor (β), [3.12] which is estimated from the slope of the FN plots in Fig 3.2.7(c).The obtained values of β for the CNT pillar array with R/H values of 6, 4.7and 1.87 are 52907, 97345 and 42901, respectively. The best field enhancement factor is obtained at R/H~4.7.
In the case for inter-distance of 250μm, the relation between R/H ratios and field-emission characteristics from the CNT pillar array are studied. Fig.3.2.9(a) shows emission current density (J) as a function of applied electric field (E) for CNT pillar arrays with heights of 18μm, 23μm, 49μm, 80μm and 100μm (R/H=14,10,5.1, 3.1,3.2,2.5and1.87,respectively). We also find that the best field-emission characteristic is obtained from the pillar array with R/H=3.1 (H=80μm), as discussed later. Fig.3.2.9(c) shows the Fowler–Nordheim (FN) plots corresponding to Fig.3.2.9(a) The occurrence of field emission is confirmed by the linearity of the FN plot. Fig.3.2.10(a) shows the Eturn-on
obtains from Fig.3.2.9(b) as a function of R/H. The optimal value of R/H giving a value of minimum Eturn-on is approximately 3.1, which corresponds to H=80μm.When R/H is smaller then 3.1, the field-emission characteristic is poor, which is likely to be due to the field screening effect.However, when the R/H is greater than 3.1, Eturn-on increases, and the reduction of field enhancement owing to a small H is conjectured. Fig.3.2.10(b) shows R/H versus the field enhancement factor (β), [3.12] which is estimated from the slope of the FN plots in Fig.3.2.9(c). The obtained values of β for the CNT pillar array with R/H values of 10, 3.1and 2.5 were 58223, 89427 and 44792, respectively. The best field enhancement factor is obtained at R/H~3.1.
Based on these results and measurements, we find not all of CNT pillar arrays having optimal R/H value of 2.[3.4] The inter-pillar distance is an important key about what optimal R/H value we can achieve. Once the inter-pillar distance is close, the edges of the pillars significantly interfere with field enhancement.[3.5] Most of field emission electrons are contributed from the edges of each pillar. One the contrary, large inter-pillar distance leads less field screening effect and each pillar can be regarded as a single emitter. Fig.3.2.11 shows that the optimal R/H values decrease from 8 to 3.1 when the inter-pillar distances increase from 80μm to 250μm. [3.10]Therefore, the optimal R/H value approaches the theoretical calculated value (i.e. 2) at larger R.
Luminescent uniformity is another important topic for backlight application. In order to improve uniformity in a unit, less field screening effect for the edges of pillar to pillar is needed. [3.6] Table3-2 shows the relation between luminescent uniformity and R/H value of different inter-pillar distances at the applied field of 2.5 V/μm. It is evident that luminescent uniformity has strong relation to the R/H value. The luminescent image has better uniformity when the R/H value is optimal for three different inter-pillar distances. The best luminescent uniformity is achieved with R/H value of 3.1 when R is 250μm. Therefore, we have demonstrated that CNTs′ pillar arrays has deep potential in the backlight application.
3.3 Analysis of Two-Step Growing Method
3.3.1 Effect of evolution recipe on the morphology of CNTs’ growth
Growth of longer CNTs seems not to be an easy job and needs to find a precise control of some parameters such as the choice of catalyst, the growth condition and growth methods, and so on. In our experiment, we have tried a lot of methods using thermal chemical vapor deposition (T-CVD) to grow higher and perpendicular pillar arrays to meet the required R/H value we have designed. A funny and special method (two-step growth) is found after numerous trials of changing growth parameters for CNTs′
growth and is proved to be an efficient way to improve field emission characteristics. In this two step growth method, the first growth step which is the same as the previous
mentioned method and the growth time is 20 min. Then, we try to control nanoparticles at the tip of CNTs not to be encapsulated completely by amorphous carbon. The cooling step is replaced by another pre-treatment process. Then, the second growth step is proceeded to flow carbon source (C2H4) at 60sccm, hydrogen at 10sccm, and nitrogen at 1000sccm into the furnace to grow CNTs for 20 min, respectively. It can keep growing until running out of activity of catalysts. Then cooling step is the same as first growing step.
Two-step growth method is an efficient way to grow longer CNTs. We have successfully used this method to obtain aligned, vertical and longer CNTs pillar arrays and some useful material analyses used to study their properties of CNTs will described in the next section.
3.3.2 SEM TEM and Raman spectrum analysis
SEM, TEM and Raman spectrum are used to study the morphology and material properties. Fig 3.3.1(a) shows CNTs′ pillar arrays with the height of about 80μ m grown by one-step growth method. Fig 3.3.1(b)-(c) show that the diameter of CNTs grown by one-step growth method ranges from 32μ m to 42μ m. Fig 3.3.2(a) shows aligned, perpendicular CNTs′ pillar arrays with the height of about 80μ m grown by two-step growth method. This confirms longer CNTs growing ability for this two step growth method. Fig 3.3.2(b) shows top view image of one pillar. Fig 3.3.2(c) shows the tilt image on the top of pillar. Fig 3.3.2(d) shows CNTs′ pillar with one joint between the upper part and the bottom part of CNT′ pillar.
Fig 3.3.3 shows the SEM images of CNTs grown by two-step growth method. The diameter of the upper part of the pillar ranges from 20 to 26μm and the diameter of the bottom part of the pillar is about 32nm. The size of catalyst nanoparticles of the two step growth method is speculated smaller than that of one step growth method. Thinner diameter of CNT in the upper part than that of the bottom part means that CNTs′ pillars grown by this two-step growth method have higher aspect ratio.
The Raman spectra of one step growth and two-step method are plotted in Fig
3.3.4. The two main features in the Raman spectra are the D and G peaks about 1350 and 1600 cm-1, respectively. The G band corresponds to the symmetric E2g vibrational mode in graphite-like materials, while the D band is activated in the first-order scattering process of sp2 carbons by the presence of substitutional hetero-atoms, vacancies, grain boundary or other defects and by finite size effects, all of which lower the crystalline symmetry of the quasi-infinite lattice. Sharper D and G peaks are shown in two-step growth case of CNTs and the IG/ID increases from 1.5 to 1.8 which confirms that the CNTs have a highly crystalline graphite structure. The TEM images show its magnified CNTs as shown in Fig 3.3.7. It is obvious that few defects in the graphite wall.
Therefore, less field screening effect and enhancement in field emission characteristics can be achieved due to thinner diameter (i.e. high aspect ratio) and highly crystalline graphite structure.
3.2.3 Comparing usual recipe and this special recipe in electric characteristic
Fig 3.3.5-(a) shows the current density versus electric field characteristic (J-E plot) of the pillar arrays. The Fowler- Nordheim (FN) plots are shown in Fig 3.3.5-(c) and their linearity verifies the field emission phenomenon. The turn-on field(Eturn-on) and threshold field (Eth) are defined to be the field required to produce the current densities of 10 μA/
cm2 and 10 mA/cm2 are 0.1 V/μm and 0.64 V/μm, respectively. The obtained Eth is the lowest value that has been reported to date. The electron field emission is monitored by a fluorescent screen at an electric field of 0.66 V/mm, as shown in Fig 3.3.6. The emission uniformity and high brightness are clearly evident.
3.4 Field Emission and Uniformity Improvement by Plasma post-treatment
3.4.1 Effect of plasma post-treatment on CNTs’ pillar morphology
Fig 3.4.1. shows the typical top view SEM images and cross-sectional views of CNT films before and after pure O
2 plasma treatment. The O
2 gas flow rate was 40sccm and the 20W BIAS Power with different etching time: (a) 2 min, (b) 10 min. The density of the CNTs decreases as plasma treats, which results from the destruction of CNTs during oxygen plasma treatment. We got the result that no matter what post-treatment with 40sccm O
2 plasma for etching time 2 or 10 min, their morphology was become short, dispersed and surface was covered with amorphous carbon and oxygen carbonization.
Thus, we tried to decrease oxygen flow to 30sccm and introduced 10sccm chlorine, respectively. The Cl2 mix O2 plasma with 20W BIAS Power with different etching time:
(a) 2 min, (b) 3min, (c) 5min and (d) 10min, were shown in Table 2-1. Fig 3.4.3 shows the SEM mocrographs of the as grown CNT pillar array was not obviously different with untreated case in macro view. But, in the micro view at the tip of the pillar, the CNTs merged together to decrease field screening effect, and increase field emission sites. Fig 3.4.4 shows the SEM images for 3min case. The result not only maintained the benefits of the Fig 3.4.3 (2 min), but also formed the ring edges around the periphery at the tip of a pillar. [3.8] This structure can provide more many field emission sites in edge areas.
[3.9] Fig 3.3.5 shows the SEM mocrographs of the as grown CNT pillar array as well as the conical shaped CNT pillars after the plasma treat for 5 min.
3.4.2 TEM, Raman spectrum and EDS analysis
We had analyzed CNT pillar array after O2 mix Cl2 plasma post-treatment by Raman spectrum, EDS and TEM analysis. The Raman spectra of the CNT pillar array before and after plasma post-treatment in the frequency range of 500–2500 cm−1 are shown in Fig 3.4.7 . The spectra show mainly two Raman bands at 1350 cm−1 (D band) and 1580 cm−1 (G band).
In the spectra of CNT pillar array, the D band of the sample after 5 min plasma treatment appears as a small shoulder of the G band at 1610 cm−1. The origin of the D and post-treatment of D bands have been attributed as the disorder features of graphitic sheets [3.7]. The D bands become stronger and sharper after plasma treatment. The details of the Raman analysis is summarized in Table.. The intensity ratio of the G band to the D band (I G/I
D) from 1.47 to 0.82 after plasma treatment.The ratio values given in Table 3-3 are the I G/I D
ratio of the corresponding peaks. We found that I G/I D ratio decayed linearly as increasing the time for plasma post-treatment as shown in Fig 3.4.8.
The EDS spectra analysis show in Table3-4. Fig.3.4.9 (a)-(d) show the EDS spectra of the tips after 2, 3, 5 and 10 min for plasma treatment, respectively. The nanoparticles observed at the tip of the aligned CNTs consisted of Co, O, Cl principally..In spite of Ti co-deposited in the catalyst, there are none of Ti atoms at the CNTs’ tips in Table3-4. It indicated that titanium (Ti) doesn’t catalyze action. However, the coble atom(%) was decayed from 0.23% to 0.14%.
It means O2 mix Cl2 plasma is effective to remove nanoparticles from the tips of the aligned CNTs. Thus, after this post-treatment we got more many emission sites at the CNTs’ tips.
The TEM images show its magnified CNT as shown in Fig 3.4.12. It is obvious that cut tip without nanoparticale as post-treatment. Therefore, it contributed more emission sites for field emission. Thus, we can got more uniformity field emission result by this way.
3.4.3 Improvement of Uniformity and FE characteristic
Fig 3.4.10 show the results of the field emission measurement made on the as grown CNT pillar array after plasma treatment. The turn-on fields (Eturn-on defined at an emission current density of 10 μA/cm2) for the sample before and after 2, 10 min O2 plasma treatment are 3.45, 3.82 and 3.94 V/μm, respectively shown in Fig 3.4.10(a). According for pure 40sccm O2 plasma consuming carbon graphite was too fast to remain some amorphous carbon and some carbon oxide. Those materials reduced field emission current and raise the turn-on filed on the contrary. In O2 mix Cl2 plasma post-treatment, we replaced 10sccm O2 to 10sccm Cl2 flow. Utilizing O2 reacted with carbon to produce CO2(g) and CO(g),and exposed the coble nanoparticles at the tip of aligned CNTs. Then, chlorine reacted with coble nanoparticles to
Fig 3.4.10 show the results of the field emission measurement made on the as grown CNT pillar array after plasma treatment. The turn-on fields (Eturn-on defined at an emission current density of 10 μA/cm2) for the sample before and after 2, 10 min O2 plasma treatment are 3.45, 3.82 and 3.94 V/μm, respectively shown in Fig 3.4.10(a). According for pure 40sccm O2 plasma consuming carbon graphite was too fast to remain some amorphous carbon and some carbon oxide. Those materials reduced field emission current and raise the turn-on filed on the contrary. In O2 mix Cl2 plasma post-treatment, we replaced 10sccm O2 to 10sccm Cl2 flow. Utilizing O2 reacted with carbon to produce CO2(g) and CO(g),and exposed the coble nanoparticles at the tip of aligned CNTs. Then, chlorine reacted with coble nanoparticles to