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Chapter 4:

Work function engineering

4.1. Background

4.2. Lanthanum hexaboride nano-particles

decoration

4.3. CNTs modified with cesium iodide nano-

particles

4.4. Summary and Conclusions

4.5. References

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Chapter 4: Work function engineering

4.1. Background

Carbon nanotubes (CNTs) are considered to be one of the best electron field

emitters compared to the other materials and Spindt type micro-emitters, owing to

their physical geometry, high aspect ratio, extraordinary electrical properties [1, 2],

good mechanical strength and high thermal conductivity. A lot of research work has

been carried out on the development of CNTs as a strong electron field emitter source

[3, 4] with high current density. However, very few commercial products based on

CNTs are available yet due to low observed current throughput. CNTs are excellent

field emitters and there still exists scope for further improving their electron emission

using various experimental approaches. This could be made feasible after

understanding the dependency of emission current on the field emission (FE)

parameters and trading these variables using different techniques [5, 6].

Introductory theories of electron emission techniques were reviewed and

analyzed earlier in chapters 2 with more emphasis on quantum mechanical FE

tunneling. Quantum mechanical FE tunneling predicted by Fowler-Nordheim [6, 7]

suggested that, emission current density of the CNT emitter arrays (CEAs) are

critically dependent and influenced by the work function [8] and applied electric field.

Applied electric field is linearly dependent on the field enhancement factor as

� = ��. Having described the different experimental techniques employed for

development of the CNT based cathode in section 3.2, furthermore discussion on the

different characterization techniques used for quality and morphology assessment of

the developed CEAs in section 3.3 and its FE measurement in section 3.4 of chapter 3.

Form this chapter onwards; different experiments were carried out on the developed

CEAs to enhance the emission current density. These CEAs can be used as an

electron source in place of the existing thermionic cathodes.

Owing to the geometry i.e. high aspect ratio (height/diameter), CNTs

inherently keep the merit of very high field enhancement factor. Further increase in

field enhancement factor will require optimization of growth parameters to obtain


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CNTs with more height and lesser diameter. Other than this, field enhancement factor

can also be altered or traded with variation in the structural parameters such as

patterned growth of CNTs, variation in CNTs dot pattern on cathode plate, cathode to

anode spacing in diode assembly, etc. This structural trading part of the CEAs as

cathode plate in diode structure is discussed in chapter 6. Second approach involves

work function trading of CNTs and for this post growth treatments can be tried. Post

growth treatment includes surface modification with different low work function

material and thermal engineering to remove residual adsorbed gas particulate and

other impurities. CNTs are reported to have work function close to graphite [8] and

this associated high work function (~ 4.5eV) limits their FE capabilities. Present

chapter is devoted to the work function engineering of the developed CEAs. Work

function engineering is expected to enhance the emission efficiency of the CEAs and

these emitters can be used as electron emitter source for high current density

application.

Work function engineering is alteration of effective work function of the

material using various techniques such as surface modification, temperature treatment

etc. Effect of temperature treatment on the FE of the developed CEAs is discussed in

the chapter 5. However the work function engineering using surface modification

approach will be employed and discussed here in the present chapter. Surface

modification is a simple and effective method that can be employed to alter the

surface density of states which attributed to the change in surface phenomena and

surface effect. This chapter is intended to report the experimental research work

carried out on surface modification of CNT cathode to reduce their effective work

function and the detailed analysis to confirm the change in work function of CNTs

using mathematical reverse engineering. The effective work function of pristine CEAs

was modified by decorating them with a low work function material using both

conventional radio frequency (RF) sputtering and thermal evaporation. FE

measurements of pristine CNTs and low work function material decorated CNTs were

carried out in diode structure at a constant inter-electrode separation of 500µm.

Initially, work function and their different modification approaches reported in

literature is discussed and later on experimental process with results and discussion

will be described.


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High emission current of CNT field emitters were reported to be achieved by

various researchers using different techniques such as a growth of vertically aligned

CEAs that was used for local field enhancement, a patterned array of CNTs was

grown for screening effect reduction, CNTs were doped with different elements and

surface of the CNTs was coated with a lower work function material and nano-

particles (NPs) etc. Furthermore, to improve the FE of CNT based electron emitters

by surface modification, various physical and chemical modification techniques have

also been explored.

Different materials have been explored for surface modification of CNTs

using various techniques like doping with transition elements, coating of CNT emitter

surface with a lower work function material and NPs etc. Liu et al [9] have shown

improved FE of double walled CNTs (DWCNT) by ruthenium (Ru) metal nano-

particle decoration using chemical method. In their work, the DWCNT surface was

modified with Ru nano-particle by a chemical method. Wadhawan et al [10] had

presented effect of cesium (Cs) metal to enhance FE of single walled CNT bundle.

Jihua et al [11] improved emission current of CNT field emitter by hafnium coating

and its annealing. In the current work, CNTs have been modified with lower work

function materials lanthanum hexaboride (LaB6) and cesium iodide (CsI). Section 4.2

elaborates effect of LaB6 NPs decoration on the patterned CEAs whereas section 4.3

discusses cesium iodide coating.

4.2. Lanthanum hexaboride nano-particles decoration

LaB6 is a traditional low work function material and have well established

industrial applications like filaments of thermionic cathodes for work function

lowering in different applications. Moreover, LaB6 were also successfully coated on

conventional tungsten filaments and silicon field emitters for work function lowering

and to improve their electron emissivity. Similar to conventional tungsten filaments,

low work function of LaB6 could be used to tailor with high aspect-ratio of CNTs. It is

expected to contribute towards the enhanced FE. To the best of my knowledge, only

Wei et al [12] have modified single multi walled CNT-emitter tip with LaB6, rather

than CEAs that can be used for high current density device development. They


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reported reduction in the turn-on electric field and improvement in the current density.

Maximum current observed by them was 70µA at a field of 1.2V/µm. However, as

per our knowledge, no one has studied the behavior of LaB6 as NPs and decorated

them to improve the FE. Only one report is available on the LaB6 tip modification

approach therefore it needs a further study for realization of such a field emitter. In

this report, Wei et al observed improvement in the FE properties specially reduction

in the turn-on electric field and enhancement in the current density of the individual

multi walled CNTs (MWCNT) by LaB6 tip modification.

4.2.1.Experimental process

CEAs were synthesized on n-type <100> silicon substrate that had patterned

arrays of iron catalyst using thermal chemical vapor deposition same as the one

discussed in chapter 3. The arrays dot pattern was 10µm × 10µm square openings

with 10µm masking between two adjacent square openings and was repeated within

200µm × 200µm square with each such square was spaced 300µm apart. The pattern

was confined within a 10mm diameter circle such that the above substrate contained

30336 open square dots and had total device area of 0.785cm2.

Scanning electron microscope (SEM, NanoEye SNE1500M and ZEISS EVO

MA15), TEM (FEI TecnaiG2), energy dispersive x-ray spectroscopy (EDS Oxford

INCA), x-ray photoemission spectroscopy (XPS) and Raman characterization

(Ranishaw inVia, air cooled argon ion laser, 514.5nm) were used for physical

morphology, elemental composition and quality assessment of synthesized CNTs. FE

measurement of the pristine CNTs was carried out in diode-structure same as the one

outlined in chapter 3.

Initially, same as iron catalyst optimization, DekTec profilometer was used for

sputter rate optimization on the smooth polished silicon substrate. Calculated sputter

rate of LaB6 was 3.5nm per minute at the same RF power of 300W without substrate

heating. A LaB6 target of 99.5% purity (FHM International) was used for decorating

the CNTs at optimized RF power of 300W in argon plasma for 35 seconds. After FE

measurement, CNTs grown silicon substrate sample was removed from copper disc

and silver epoxy was cleaned from the back surface of the substrate. Sample was then


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loaded in RF sputtering system (Nordiko, NM1500) for LaB6 deposition. The

presence of LaB6 on the CNTs, their physical morphology with elemental signature

and defect analysis were carried out using SEM, TEM, EDS and Raman

characterizations.

FE of the LaB6 decorated CEAs were studied under same conditions as earlier.

4.2.2.Results and Discussion

Physical morphology of CNT emitters before LaB6 decoration was analyzed

from the SEM micrographs shown in Figure 4.1(a) and Figure 4.1(b). The CEAs had

slightly non uniform growth with some flower like structure at the apex and also with

few CNTs spreading outside the patterned dot. These micrographs confirm the

patterned growth of CNTs in the 10µm × 10µm dot with 10µm inter dot spacing

within the square area of 200µm × 200µm. The SEM micrograph of same sample

after FE measurement and after LaB6 decoration is shown in Figure 4.1(c). The

quality of SEM images before and after LaB6 deposition appears to be same. Any

morphological change or damage such as peel off, substrate melting etc is not

observed. However, this micrograph also does not provide clear insight of the LaB6

decoration and modification pattern over the CNTs top surface as to whether sputtered

LaB6 was deposited or not. A low deposition time of about 35 seconds was kept to

decorate the CNTs with monolayer of LaB6 rather than thick film. To achieve the

enhanced emission performance of the modified CNT emitter, LaB6 NPs or

monolayer deposition was preferred over thick or continuous film. Continuous thick

film coating on the CEAs may completely cover their top morphology and makes

them equivalent to a flat surface. This effectively diminishes many advantages offered

by CEAs as strong field emitter. Calculated thickness of sputtered LaB6, from

optimized sputtered rate, was 2nm, and it is very thin to detect in SEM with such a

low magnification. Moreover, LaB6 target show intense purple color as a bulk

material but it appears transparent on glass when approximately 200nm thick film was

deposited.


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Figure 4.1 SEM micrographs of (a) pristine CEAs confined within a 200µm × 200µm

square, (b) magnified image of pristine CEAs and (c) LaB6 decorated CEAs confirm

the selective and patterned growth of CNTs within the 10µm × 10µm square with

10µm inter dot spacing.

Morphological differences between bare and LaB6 decorated CEAs were

analyzed by TEM and results are shown in Figure 4.2. From TEM micrograph of bare

CNTs shown in Figure 4.2(a), it is observed that CNTs have a range of diameter


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distribution and are like a hollow bore having approximately 5-25nm inner diameter

and 35-50nm outer diameters. The walls of CNTs were smooth and this signifies that

CNTs surface was not modified with LaB6. Figure 4.2(b) and 4.2(c) show the bright

field and dark field TEM micrographs of LaB6 decorated CNTs. This confirms the

coating of LaB6 as NPs, rather than continuous film, over the entire CNT strands. In

bright field TEM micrograph, NPs look like a black spot spreading on the walls of

CNTs where as in dark field imaging these NPs appear as white dot uniformly

distributed over the CNTs strands. One more remarkable point observed in the

micrograph was the absence of black or white spot away from the walls of CNTs.

This clearly signifies and confirms that NPs were not found scattered and was firmly

adsorbed on the surface of CNTs. In other words, good adherence of LaB6 NPs with

CNTs was observed.

Subsequently, magnified TEM images were taken to investigate the

topological and NPs size distribution of bare and LaB6 decorated CNTs. The

micrograph of bare CNTs is shown in the inset of Figure 4.2(a) whereas Figure 4.2(d)

and inset of Figure 4.2(b) represent the LaB6 NPs decorated CNTs. These

micrographs confirmed that synthesized CNTs were multi-walled and had variable

wall thickness. Morphology of individual wall was clearly visible at the squeezing

point of Figure 4.2(d) and this was highlighted as encircled area. Same as low

magnification TEM micrograph, smooth walls of bare CNTs were compared with

dark black and light black spots visible on the walls of LaB6 decorated CNTs. It again

affirms the deposition of LaB6 as nano sized particles and these NPs have a range of

particle size distribution from 2-12nm. Marked differences in the contrast of sputtered

LaB6 NPs were observed. This contrast difference exhibited presence of heavy

element with some light element. Lanthanum (La) is a heavy metal and its

transparency is comparatively poor than light element boron (B). Therefore, dark

black spot specify the presence of lanthanum metal whereas light black spot signifies

the presence of boron.


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Figure 4.2 TEM micrographs of (a) bare CNTs, (b) LaB6 NPs decorated CNTs, (c)

dark field TEM image of LaB6 NPs decorated CNTs and (d) magnified TEM image of

LaB6 coated CNTs. Inset shows the magnified TEM images of bare CNTs and LaB6

NPs decorated CNTs.

Here, LaB6 NPs formation procedure was interpreted or envisioned to be

governed by non uniform and rough top surface morphology of CNTs. Morphology of

sputtered material is highly influenced by substrate smoothness, temperature and

evaporation power. Our sputtering procedure is a non reactive physical vapor

deposition of compound target (LaB6) in presence of argon plasma. Sputtered LaB6

molecules or atoms, created in argon ion bombardment during sputtering process,

does not found smooth surface, as in plane silicon substrate used for sputter rate

optimization, to make the continuum and sit like a NPs rather than film. Thus, non

uniform and rough surface of CEAs was supportive in NPs formation. Decoration of

NPs has developed nano protrusions at the walls and top surfaces of CEAs. These

protrusions effectively creating nano emission sites which are expected to improve the

emission uniformity and current density of CNTs apart from work function lowering.

(b) (a)


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Therefore, rough surface of CEAs are beneficial in NP decoration as well as FE

enhancement.

Figure 4.3 EDS spectra of the (a) bare CEAs and (b) LaB6 NPs decorated CEAs.

Carbon (C), oxygen (O) and silicon (Si) signatures were observed for bare CNTs

whereas additional lanthanum peak was noticed in the EDS spectrums of the LaB6

decorated CNT emitter and light element boron was not detected.

Compositional differences between bare and LaB6 NPs decorated CNT

emitters were studied using EDS and its spectrums are depicted in Figure 4.3. Only

carbon, oxygen and silicon peaks were observed for bare CNTs, presented in Figure

4.3(a), whereas an additional peak of lanthanum was seen for LaB6 decorated CEAs,

shown in Figure 4.3(b). The EDS spectrum of the bare CNTs was captured vertically

on the top surface of a single dot. However due to the low thickness of LaB6 NPs, it is

necessary to take the EDS spectrum of LaB6 decorated CEAs at the grazing angle to

detect the large number of surface atom or change in top surface morphology.

Difference in the relative peak intensity of silicon, carbon and oxygen peaks was

attributed to the change in detection angle of bare CNTs and LaB6 decorated CNTs.

Due to low florescent yield, overlapping peak with carbon and fewer amounts of

boron than carbon, boron peak was not observed.

XPS analysis was carried out to affirm the presence of LaB6 NPs on the CEAs

and to study the reaction and contamination of LaB6 NPs with CNTs. Initially bulk

LaB6 target was characterized and its peak position is given in Figure 4.4(a).


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Although XPS peak position of the bulk LaB6 were well reported and cited, however,

to further confirm the quality of the sputtered target and the peak position of the

deposited material and later to correlate it with the deposited LaB6 NPs on the walls

of CNTs, bulk LaB6 XPS analysis were performed. This spectrum confirms the

material quality and gives information about the sharp peaks of lanthanum near 106.5,

197, 837 and 853 eV. The existence of boron in the target material is also confirmed

from the sharp but relatively weak peak of boron at 188. Boron exhibit low atomic

number and its sensitivity is weak compare to the heavy element lanthanum. In

addition to the LaB6 composition, target also contains oxygen and carbon as

contamination and its signature is observed at 531.5 and 285.5eV. Sputtering and

cleaning of LaB6 target with the argon ions in different runs for 5 mins reduces the

peak intensity of these two elements however, they are not removed completely. The

presence of carbon and oxygen in the LaB6 target after argon sputtering predicted that

these light elements exist and trapped in the voids of the boron cage and lanthanum

atom and they are not removed completely after rigorous argon sputtering.

The XPS spectrum of the LaB6 NPs decorated CEAs is shown in Figure

4.4(b). The LaB6 NPs decorated CEAs exhibit sharp peaks of lanthanum near 106,

837.5, 853.5eV and a weak, broad, shifted peak of boron is observed at 197eV.

Silicon signature observed near 100 and 151eV is attributed to the silicon substrate of

CEAs. Other defect peaks such as nitrogen and argon is observed near 398eV and at

242eV respectively. These element signatures are observed due to presence of

ammonia during growth and argon plasma during sputtering. A prominent peak of

oxygen is observed near 531eV. Apart from EDS, high intensity peak of oxygen in

XPS spectra also confirm that oxygen content in LaB6 NPs decorated CEAs is

relatively higher.


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Figure 4.4 XPS spectra of the (a) bulk LaB6 and (b) LaB6 NPs decorated CEAs.

To further characterize the LaB6 NPs crystallography, x-ray diffraction (XRD)

experiment was performed between Bragg angle of 0o to 60o and its spectrum is

presented in Figure 4.5. Figure 4.5(a) and 4.5(b) show XRD spectrum of bulk LaB6

and LaB6 NPs decorated CEAs. Bulk LaB6 exhibited different peaks of LaB6 at

different Bragg angles. However, signature of only one peak is observed in LaB6 NPs

coated CEAs. Inset of Figure 4.5(b) shows this peak position. This was attributed to

the low thickness of sputtered LaB6 NPs and the other possible reason may be the


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amorphous nature of LaB6 NPs. Silicon peak was also not observed due to forbidden

reflections of <100>, <200> and <300> planes and the Bragg angle of <400> plane

lies at 69o.

Figure 4.5 XRD spectra of the (a) bulk LaB6 and (b) LaB6 NPs decorated CEAs.

Furthermore, non destructive optical characterization on the bare and LaB6

NPs decorated CEAs to affirm the presence of CNTs and change in their chemistry

after NPs decoration. Raman spectra of the bare and LaB6 NPs decorated CNTs

presented in Figure 4.6, show defect peak, graphitic peak and second over tone of


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defect peak at 1356cm-1, 1578cm-1 and 2700cm-1 respectively which is known as D-

band, G-band and G'-band. The prominent G-band indicates well graphitized CNTs

while D-band is indicative of defects. Silicon peak was also observed at Raman shift

of 521cm-1. The marked difference in the Raman spectra of bare and LaB6 NPs

decorated CNT emitters was enhancement in Raman intensity and improvement in

relative D-band to G-band intensity (ID/IG) ratio. LaB6 NPs decoration on the CEAs

significantly added more defects. This attributed to the improvement in ID/IG ratio of

the bare CNTs to the LaB6 NPs decorated CNTs from 0.62 to 0.86. In general, it is

considered that with increase in defects FE capability of CNT emitters diminishes.

But the added defect sites on the CNT emitters effectively creates nano protrusion and

this may improve FE and emission uniformity, as reported by Liu et al [9] in case of

Ru NPs coating. From the improved Raman intensity of LaB6 NPs decorated CEAs, it

can be proposed that CNTs emitter arrays exhibited surface enhanced Raman

scattering due to adsorbed LaB6 NPs on their walls.

Figure 4.6 Raman spectra of the bare and LaB6 NPs decorated CEAs exhibited D-

band at 1356cm-1, G-band at1578cm-1 and the G'-band at 2700cm-1

Figure 4.7 (a) and (b) shows the comparison of current density-electric field

(J-E) and Fowler-Nordheim (F-N) plots of CEAs with and without LaB6 NPs. Straight

line nature of F-N curves confirms the electron emission is by quantum mechanical

FE tunneling while J-E curves show typical vacuum diode characteristics. Emission


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current density of CEAs improves more than six-fold from 2.05 to 13.19mA/cm2 due

to LaB6 NPs decoration at a constant applied electric field of 4.6V/µm. Remarkable

reduction in the turn-on electric field is also observed from 3.0 to 2.1V/µm for bare

CNTs to LaB6 NPs decorated CNTs. Here, turn-on electric field is the applied electric

field at a constant emission current of 100µA. This increase in the current density and

reduction in turn-on electric field was attributed to be caused by the lowering of

effective work function and creation of new emission sites due to LaB6 NPs

decoration or improvement in field enhancement factor. Furthermore, significant

changes in F-N slope after LaB6 NPs decoration on CEAs was observed and the

results were compared to the bare CEAs. Intercept of the F-N plots on y-axis appears

to converse at single point with small but significant difference in intercept point.

Slope of the F-N plot is linearly related to the work function and field enhancement

factor thorough the equation (2.12). This also signifies that variation in slope may be

attributed due to the change in either one or both of FE parameters.

Figure 4.7 Comparison of (a) J-E plots and (b) F-N curves of bare and LaB6 NPs

coated CEAs. Current density enhanced more than six-times at a constant applied

electric field of 4.6V/µm.


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4.2.3.Analysis of FE data

To ascertain the possible reason attributed to the improved FE of CNT

emitters, change in work function and field enhancement factor of bare CNT emitters

and LaB6 NPs decorated CNT emitters were estimated. Different methods have been

adopted by various research groups to calculate the work function of CNTs using

various techniques and had suggested a range of work function values. They observed

wide discrepancy in the work function values of CNTs due to the ambiguity in the

estimation of effective emitting area and the local field enhancement factor.

Moreover, other parameters like CNTs electronic band states, closed and open cap

CNTs, adsorbents lying at the surface of CNTs [13, 14] etc. highly influence their

work function. Therefore, precise work function of pristine CNTs is not well

identified [8]. To eliminate this from present work, only work function ratio of bare

CNT emitters to the LaB6 NPs decorated CNT emitters were evaluated using intercept

and slope of F-N plots.

The calculated field enhancement factor using equation (2.13), F-N slope and

reported work function of 4.5eV was 3308 and 4630 for bare CNT emitters and LaB6

NPs decorated CNT emitters. The increased field enhancement factor attributed to the

discrepancy and doubts as to whether improved FE was due to the change in work

function or field enhancement factor. As quoted earlier by [8] and in equation (2.13),

the estimated work functions of CNTs are strongly dependent on field enhancement

factor and was highly influenced by change in surface morphology or modification

[14-16]. Since, CNTs surface were modified with the LaB6 NPs and was the only

change that had introduced earlier. Therefore, improved current density and reduced

turn-on electric field was only attributed to the change in work function rather than

field enhancement factor. Furthermore, FE comparison was also done on same sample

before and after LaB6 decoration, hence two important parameters, field enhancement

factor and effective emitting area, were approximated constant.

Substitution of field enhancement factor � in y-axis intercept of equation

(2.13), reported in section 2.8 of chapter 2, and their rearrangement give the work

function value of electron field emitter as given in equation (4.1).


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(4.1)

Therefore, change in work function due to LaB6 NPs decoration was evaluated

by taking work function ratio of the bare CNT emitter to the LaB6 coated CNT emitter

as given in equation (4.2),

(4.2)

F-N slopes and y-axis intercepts, obtained from Figure 4.7, were substituted in

the above equation (4.2) which gives work function ratio of 1.68 and show

remarkable percentage reduction of 40.48% in the work function of LaB6 NPs

decorated CNTs. To determine the work function of LaB6 NPs decorated CEAs,

reverse engineering were done by dividing the reported work function of 4.5eV with

the calculated work function ratio. The estimated work function was 2.68eV which is

approximately close to the well reported work function value of bulk LaB6. This

signifies that LaB6 nano protrusions are major or sole contributor to the improved

emission rather than CNTs. This also confirms that LaB6 NPs retain their low work

function property and are only adsorbed on the CNTs surface which reduces effective

work function of CEAs. Therefore, reduced effective work function and increased

nano emission sites are major causes of remarkable improvement in FE of LaB6 NPs

decorated CEAs.

To compare the observations with other reports on surface modification

techniques, this approach intended to be more simplistic for industrial applications

because it does not require any chemical treatment, annealing steps and does not react

or contaminate CNTs. Although, Wei et al [12] finding on CNTs and Wang et al

report on silicon-tip are in agreement with our observation. But, they had modified

individual multi walled CNTs and an arrays of silicon tip with continuous and thick

film of LaB6 rather than CEAs with LaB6 NPs. Emission current observed from LaB6

NPs coated CEAs was 10.36mA at a field of 4.6V/µm which is far away from Wei et


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al [12] observation. Other FE parameters like screening effect, field enhancement and

adhesion were also addressed during sample preparation by pattern transfer, aligned

and direct growth of CNTs on silicon-substrate. Therefore, LaB6 NPs decoration on

the CNT emitters can be employed as a simple and convenient method to improve the

current density of existing CNT-emitters. Remarkable improvement in the current

density of the LaB6 NPs coated CEAs indicate its great promise to serve as an

electron gun of microwave tubes [3].

4.2.4.Conclusion

As-grown patterned CNT emitter arrays were successfully decorated with

LaB6 NPs using RF sputtering to improve the emission current. More than Six-fold

improvement in the current density 'J' and significant reduction in the turn-on electric

field was observed in LaB6 NPs decorated CNT emitters. Mathematical reverse

engineering confirms that actual reason of enhanced emission current was lowering of

effective work function of CNTs due to LaB6 NPs decoration. The LaB6 nano

protrusions formed on the CNTs surface effectively creating new emission sites and

are the sole reason of improved FE. Furthermore, LaB6 NPs keeps the merit of low

work function as bulk material and their calculated work function was approximately

2.68eV. TEM and Raman study confirm that LaB6 NPs deposition added more defects

on the walls of CNTs and this contributed to the improved emission current.

4.3. CNTs modified with cesium iodide nano-particles

Same as LaB6, various other materials such as metal carbide, cesium metal,

cesium iodide (CsI) etc were also explored for effective work function lowering

material of CNTs. Few paper reports Cs metal doping in CNT-emitter to enhance the

electron emission. Though, cesium metals have a low work function (2.1eV), but it

has room temperature melting point of 280C approximately. Therefore, cesium doped

CNT-emitter exhibit may possess variable electron emissivity depending upon the

operating temperature/conditions. Each material and their coating techniques have

their merits and limitations.


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Shiffler et al [17] reported that CsI films enhances the FE properties of carbon

fibers and such coatings results in reduced out gassing as well as improved emission

uniformity. Another interesting observation made by them was remarkable reduction

in the turn-on voltage. They also concluded that emission current is highly sensitive to

the coating process. Vlahos et al [18] investigated lowering of work function using

theoretical approach by carrying out ab initio calculations of CsI molecules adsorbed

onto graphite surfaces. Wadhawan et al [10] demonstrated that cesium metal improves

the FE characteristics of the single walled CNT bundle.

To the best of our knowledge till date, no one has reported decoration of CNTs

by CsI to improve their emission performance. Enlightened by the results observed by

Shiffler et al [17, 19], patterned arrays of aligned CEAs were modified to improve

their emission capabilities by coating them with CsI NPs. In this, characterization of

CNT emitters before and after CsI-coating is presented along with investigation of

change in FE properties.

4.3.1.Experimental process

CEAs were developed on the silicon substrate using same process steps

discussed in section 3.3 of chapter 3. FE of developed CEAs were carried out under

high vacuum condition at a pressure of than 1 × 10-7 Torr using same measurement

setup elaborated in section 3.4 of earlier chapter. After FE measurement, CEAs was

removed from copper disc and the back surface was cleaned to remove the silver

epoxy. Sample was then mounted in thermal evaporation system.

Cesium iodide exists in the granular form. Thermal evaporation was used for

decorating the walls and top surface morphology of synthesized CEAs to improve the

emission current of existing CNT cathodes. A high purity (5N) CsI (2.1mg) was

loaded in molybdenum (Mo) boat and evaporated at a current of 82A over CNT

emitter surface. Few milligram of cesium iodide were used for decoration of CEAs.

Microbalance (Sartorious) was used for weighing the amount of cesium iodide.

Glossy paper was used as sample holder bowl to weight the cesium iodide.

Prior to material loading Mo-boat was fired at a current of 95A with the base

vacuum better than 4 × 10-6 Torr. During CsI-coating, sample was heated at 3000C to


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improve the adherence of CsI with CNT emitter. SEM, EDS, X-ray diffraction (XRD,

Bruker D8 Discover, Bruker AXS Inc.) and Raman characterization were carried out

to study difference in morphology, elemental composition, stoichiometric and

crystalline nature of the CEAs and CsI decorated CEAs. Immediately after CsI-

coating, FE measurements were again carried out on the CNT emitter under the same

conditions.

4.3.2.Results and Discussion

SEM micrographs of CNT emitter, shown in Figure 4.8(a) and 4.8(b), confirm

aligned and selective growth of CNTs as square bundles. CNTs were mostly confined

within the 10µm × 10µm square dots having 20µm inter dot spacing. Few CNTs are

protruding outside the square dots and those lying inside the square bundles are

entangled with each other.

Figure 4.8(a) SEM images of patterned square arrays of CNT emitters synthesized

using CVD and (b) a single dot of aligned CNTs at higher magnification.

SEM micrograph of the same sample after FE measurement and CsI-coating,

presented in Figure 4.9(a) and 4.9(b) respectively, confirm that quality of CsI-coated

CNT emitters were maintained even after the processing. CsI-coated CEAs were not

damaged during FE measurements and are strongly adhered to silicon substrate. It

does not get peeled off due to strong applied electric field during FE measurements.

Substrate melting was also not observed in the micrograph, which is reported to be


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caused due to localized Joule’s heating during FE measurement. Any other

morphological change or damage was not observed between bare CNT emitter and

CsI-coated CNT emitter. These images are captured at an inclined angle to estimate

the CNT height, uniformity and quality of coated CsI. However, these micrographs do

not give clear insight into the surface morphology of CsI-coated CNT emitters as to

whether the coating consists of thin film or NPs.

Figure 4.9(a) SEM micrograph of CsI-coated CNT emitter shows nano sized and

uniformly distributed CsI particles and (b) A single dot of CsI-coated CNT emitter.

To clarify these doubts, another SEM instrument with high magnification,

resolution and image quality (ZEISS EVO MA15) was used and its images are shown

in Figure 4.10. As reported earlier, top surface morphology of the pristine CNT

emitter again confirms that CNTs are grown like a forest inside the square bundles

and are entangled with each other. Qualitative and morphological differences were

observed on the top surface of CNT emitter before and after CsI-coating. The top

surface of CNT emitter bundles was not smooth prior to the coating and CsI NPs

deposition introduced further roughness. The roughness of CNT emitter tip surface is

beneficial to FE due to reduced shielding effect. Continuous thin film coating on the

CNT emitter completely covers their tip and makes them equivalent to a flat surface.

This effectively may reduce many advantages offered by them as strong field

emitters. Therefore, coating by NPs is preferred over continuous thin film. The

particle size of CsI NPs is approximately 40-50nm. CsI NPs were found to be


121

uniformly deposited over the entire CNT bundles including the substrate and were

clearly visible even over the stray CNT strands protruding out of the bundles. CsI NPs

deposited over the CNT square bundles have smaller size distribution and are

comparable to the CNT diameter whereas those lying on the substrate have bigger

particle size, Figure 4.10 (c).

Figure 4.10 SEM micrograph of single dot of the CNT emitters top surface (a) before

and (b)-(c) after CsI coating. CsI nano sized particles were uniformly distributed over

entire CNT bundle and substrate.

Furthermore, qualitative differences between pristine CNTs and CsI decorated

CNTs were ascertained by TEM analysis. TEM micrograph of the pristine CNT

emitters, presented in Figure 4.11(a) and 4.11(b), confirm their multiwalled structure.

CNTs have a range of diameter distribution with hollow bore like structure as

observed in our previous study. CNTs have variable wall thickness and have

approximately 5-25nm inner diameter and 35-50nm outer diameters. Decoration with

CsI NPs was not observed as walls of CNTs appear to be smooth. This signifies that

CNTs surface was not modified or decorated with CsI NPs. TEM micrographs of CsI

NPs decorated CNTs are depicted in Figure 4.11(c) and 4.11(d) respectively. The dark

black spots spreading on the walls of CNTs clearly signify the presence of CsI NPs.

This again confirms the decoration of CsI as NPs, rather than continuous film, over

the entire CNT strands. Another remarkable point observed in the micrograph was

absence of black spot away from the walls of CNTs and few of the CNTs have

bamboo like shape as observed in Figure 4.11(d).


122

Figure 4.11 (a)-(b) TEM micrograph of bare CNT emitters, (c) CsI NPs modified

multi-walled CNTs and (d) bamboo shaped CNTs decorated with CsI NPs.

Compared to the previous report, CsI NPs decoration on patterned CEAs is

completely different from the previous LaB6 NPs modification. Thermally evaporated

CsI vapor condenses on the sample and nucleates in spherical shape like water droplet

and sits like a NPs rather than thin film. The nucleation may be attributed due to the

major difference in electron affinity, electropositive nature of cesium and

electronegative behavior of iodine atom, and in other word due to polar nature of the

molecule. The other possible reason of CsI vapor nucleation as spherical droplet can

be temperature treatment of CNTs substrate at 300°C during CsI coating. CsI NPs size

distribution is relatively bigger and is comparable to the CNTs diameter. Few NPs

size was even bigger than the CNTs diameter. As quoted in previous reports, thin film

and NPs decoration is highly influenced by the non uniform and rough top surface

morphology of CNTs and is highly sensitive to deposition process. Thermal

evaporation is a non reactive physical vapor deposition used for thin film deposition


123

and morphology of the deposited film is critically influenced by substrate smoothness,

temperature and evaporation power. Thermally heated CsI in liquid or solution phase

evaporates and condenses on the rough top surface of the CNTs and did not find

smooth surface on the top surface of CNTs to make the continuum. Thus, non uniform

and rough surface of CEAs was supportive in NPs formation. Decorated CsI-NPs

created nano protrusions at the walls and top surfaces of CEAs. These protrusions

effectively developed nano emission sites which are expected to improve the emission

uniformity and current density of CNTs apart from work function lowering.

Therefore, rough surface of CEAs are beneficial in NPs decoration as well as FE

enhancement.

Qualitative composition (elemental) difference of CNT emitter and CsI coated

CNT emitters were measured using EDS and the spectrum is shown in Figure 4.12.

Pristine CNT emitter exhibits signatures of iron, carbon, silicon and oxygen only,

while CsI coated CNT emitter shows carbon, silicon and additional peaks of cesium

and iodine. Iron signatures in the uncoated CNT emitter signifies that iron particles lie

at the top surface of CNT emitters rather than at the silicon-CNT interface and from

this result we can predict that CNTs obey tip growth approach.

Figure 4.12 EDS spectrum of (a) bare CNT emitters and (b) CsI-coated CNT

emitters. Silicon, carbon, iron and oxygen peaks appear in the EDS results of pristine

CNT emitter whereas silicon, carbon, cesium and iodine peaks appear in the EDS

spectrum of CsI coated CNT emitters.


124

To further ascertain the stoichiometry and crystalline nature of the deposited

NPs, XRD analysis was performed and results are shown in Figure 4.13. CsI-coated

CNT emitter has exhibited high intensity peaks of CsI only at 27.50 and 48.760 having

Miller indices <110> and <211> orientations. Silicon <100>, <200> and <300>

planes have forbidden reflections according to selection rules and Bragg angle for

<400> plane is at 690. Although Chen et al [10] quote that diffraction pattern of the

graphitized CNTs correspond to the high intensity peak of <002> plane at 26.020 and

several weak peaks were also observed by them. But our XRD analysis on bare CNT

emitter does not show any peak as observed by them. Therefore we got a clear

background and only signatures of CsI were observed.

Figure 4.13 XRD spectrum of CsI coated CNT emitter. Two major peaks appear at

27.5° and 48.76° correspond to the high intensity peaks of CsI compound having

<110> and <211> orientation.

The Raman spectra of CNT emitters and CsI coated CNT emitter, given in

Figure 4.14, show the three characteristics peaks of the CNTs; the D-band at 1348cm-

1, the G-band at 1588cm-1 and the G'-band at 2700cm-1. The D-band is indicative of

defects in the CNTs. Since CNTs are grown by CVD, they have certain carbonaceous

impurities and broken sp2 bonds in the sidewalls. The prominent G-band indicates

well graphitized CNTs while G'-band is a second over tone of D-band. The marked

difference in the Raman spectra of CNT emitters was reduction in Raman intensity


125

after CsI-coating and this was attributed to the covering of CNTs with CsI NPs.

Qualitative difference between bare and CsI coated CNT emitters was estimated by

the relative D-band to G-band intensity (ID/IG) ratio. CsI NPs decoration on CNT

emitters significantly added more defects on CNTs and thus improved ID/IG ratio of

pristine CNT emitters to CsI-coated CNT emitters from 0.85 to 0.93.

Figure 4.14 Raman spectra of the pristine CNT emitter and CsI-coated CNT emitter

shows the three characteristics peaks of D-band at 1348cm-1, G-band at1588cm-1 and

the G'-band at 2700cm-1.

Figure 4.15 Comparison of (a) J-E plots and (b) F-N curves of pristine CNT emitters

and CsI coated CNT emitters.


126

Figure 4.15 shows the J-E and F-N curves of our typical CNT emitters before

and after CsI-coating. F-N curve is a straight line which is characteristics of electron

emission by quantum mechanical FE tunneling, while J-E curve shows typical

vacuum diode characteristics. In Figure 4.15(b) for CsI-coated CNT emitter, the F-N

curve is in good agreement with the straight-line behavior predicted by the F-N

equation and it also has uniform slope. On the other hand, pristine CNT-emitters show

slightly non-linear behavior of F-N curve and its slope is varying. From J-E plots, it is

observed that turn-on electric field was at 3V/µm prior to CsI-coating. However

significant reduction in turn-on electric field to 2.13V/µm was observed after CsI-

coating. Here, turn-on electric field is knee field at a constant current density of

100µA/cm2. After CsI-coating on CNT-emitters the emission current density also

shows remarkable improvement by more than 50%, from 11.02 to 17.33mA/cm2 at a

constant electric field of 5V/µm. Similar to LaB6 NPs decoration, CsI NPs coated

CEAs also exhibit change in F-N slope compared to the pristine CEAs with

approximately conversing intercept on y-axis.

4.3.3.Analysis

To analyze the possible reason of improved FE performance from the CsI-

coated CNT emitter, we have computed work function ratio of the pristine CNT

emitter and CsI-coated CNT emitter. Previous reports suggest that the ambiguities

exists over the precise work function, field enhancement factor and effective area

calculation of CNTs. Various other parameters such as CNT electronic band states,

closed and open cap CNTs, adsorbed molecules at the surface of CNTs highly

influence their work function. As quoted earlier, CsI NPs adsorption (decoration) on

CNT emitters added more defects which contributed towards work function

modification and enhanced FE current density. To avoid this discrepancy in our

analysis, instead of finding change in absolute values of work function for CNTs, we

have evaluated the change in work function as work function ratio for the CNT

emitters prior to CsI coating and after CsI coating.

The value of � = �� can be determined from F-N curve in Figure 4.15(b),

and it is equal to slope of F-N curve ln(I/V2) vs. 1/V . Here, 'B', 'd', 'ϕ' and 'β' are


127

exponential factor, anode emitter separation, work function and field enhancement

factor respectively. It can be seen that there is marked difference in slope of F-N

curves for CNT-emitters before and after CsI deposition while their intercepts on the

y-axis is approximately constant. FE parameters like field enhancement factor, work

function and effective emitting area are intimately related to each other through

experimental F-N slope and intercept on y-axis. Since FE measurements were carried

out on the same sample having same geometry and under same measurement

conditions, therefore we can approximate field enhancement factor 'β' remains

unaltered even after CsI NPs deposition. But there can be marked change in effective

work function of CNT emitters due to surface modification by CsI NPs coating and

this change in work function has contributed to reduction in slope of F-N curve. This

change in effective work function as ratio of pristine CNT emitter to the CsI-coated

CNT emitter can be calculated from the ratio of their respective F-N slopes as given

in equation (4.3).

� ��

� �� = � � ��

� ��

(4.3)

Substitution of slopes from the F-N plots in equation (4.3) gives the work

function ratio equal to 1.54 for the pristine CNT emitters to the CsI-coated CNT

emitters. This lowering in the effective work function of CsI coated CNT emitter is

therefore considered responsible for the remarkable enhancement in their current

density and significant reduction in the turn-on electric field. Reduction in the turn-on

electric field and increase in the FE current is either due to tunneling from CsI

adsorbate states or due to reduction in effective work function at the surface of the

CNTs, or a combination of both.

As compared with CNT emitter modified by traditional materials such as

titanium, zirconium, hafnium, LaB6 etc., this approach does not need conventional

annealing steps [12] complex chemical treatment [9] and does not react with the

CNTs [3] as in the case of zirconium carbide and titanium carbide due to the joules

heating. From our XRD results, chemical states of the coated material (CsI) were

maintained even after thermal evaporation, as also observed by Fairchild et al [4]. Our

work of decorating the CsI NPs over the CNT emitters is more simplistic for device


128

fabrication. Therefore for practical and industrial application, CsI NPs decoration can

be employed as a simple, feasible and convenient method for improving the current

density of existing CNT-based cathodes.

4.3.4.Conclusion

CsI NPs coating on the CNT emitters has significantly improved the FE

properties, such as reduction in the turn-on electric field by a factor of 1.4 and

increase in the current density by more than 50%. Additionally, the ratio of work

functions after and before CsI NPs deposition is approximately 0.65 (1/1.54),

indicating reduction in the work function of the CsI-coated CNT emitter. Our present

work of CsI coating on the CNT emitter can be used as a simple and effective method

for improving the current density of existing CNT emitters, especially for electron

source applications which require very high current densities.

4.4. Summary & Conclusions

Work function engineering is a robust and effective approach to enhance the

electron emissivity of the CEAs. LaB6 and CsI were successfully decorated on the as-

grown patterned CEAs as NPs rather than continuous film using RF sputtering and

thermal evaporation respectively. LaB6 NPs decorated CEAs exhibited remarkable

improvement of more than six-fold whereas CsI NPs coated CEAs show robust

enhancement of approximately two times in emission current density. Significant

reduction in the turn-on electric field was also observed for both the LaB6 NPs and

CsI NPs modified CEAs. The decorated LaB6 NPs have smaller particle size

distribution that ranges from 2-12nm. However, CsI NPs size distribution is bigger

than LaB6 NPs and the CsI particle sizes are comparable to the diameter of CNTs and

even bigger than CNTs diameter. The particle size of CsI NPs is approximately 40-

50nm. One more remarkable point observed form the comparison of XRD spectrum

of bulk material and decorated NPs is that decorated NPs obey preferred orientation.


129

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