Flexible carbon nanotube-array cathodes: Fabrication and bending effect on field-electron emissionNguyen Tuan Hong, Ken Ha Koh, Soonil Lee, Ngo Thi Thanh Tam, and Phan Ngoc Minh Citation: Journal of Vacuum Science & Technology B 28, C2C5 (2010); doi: 10.1116/1.3363855 View online: http://dx.doi.org/10.1116/1.3363855 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/28/2?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Solid-state fabrication of ultrathin freestanding carbon nanotube–graphene hybrid structures for field emissionapplications J. Vac. Sci. Technol. B 32, 06FF03 (2014); 10.1116/1.4899241 Flexible field emitter arrays with adjustable carbon nanotube distances and bundle generation J. Vac. Sci. Technol. B 28, 268 (2010); 10.1116/1.3298889 Field-electron emission from flexible carbon nanotube array cathodes J. Vac. Sci. Technol. B 27, 753 (2009); 10.1116/1.3072831 Field electron emission from free-standing flexible PDMS-supported carbon-nanotube-array films J. Vac. Sci. Technol. B 26, 778 (2008); 10.1116/1.2827503 Fabrication and field emission characteristics of high density carbon nanotube microarrays J. Vac. Sci. Technol. B 23, 772 (2005); 10.1116/1.1880132

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Flexible carbon nanotube-array cathodes: Fabrication and bending effecton field-electron emission

Nguyen Tuan Hong, Ken Ha Koh, and Soonil Leea�

Division of Energy Systems Research, Ajou University, Suwon 443-749, Korea

Ngo Thi Thanh Tam and Phan Ngoc MinhInstitute of Materials Science, VAST, 18 Hoang Quoc Viet, Hanoi, Vietnam

�Received 15 September 2009; accepted 22 February 2010; published 30 March 2010�

The authors report the fabrication of freestanding field-electron emitters based on arrays ofvertically aligned carbon nanotubes �VACNTs� and flexible polydimethylsiloxane �PDMS�.Transplant of VACNT arrays from silicon substrates to flexible PDMS platforms through apress-and-curing process resulted in PDMS-supported VACNT-array electron emitters. Test offield-electron emission from the PDMS-supported VACNT columns in a diode configurationshowed good field-emission results regardless of cathode geometry, either planar or convex shapecathodes. Furthermore, the repeated bending of the PDMS-supported VACNT-column cathodes upto a few hundred times showed no noticeable degradation in field emission. Numerical simulationsof electric field distribution at various bending angles and anode-cathode distance show that thegeneral trend in emission-current variations is consistent with the difference in maximum electricfield strength at the cathode surface. © 2010 American Vacuum Society. �DOI: 10.1116/1.3363855�

I. INTRODUCTION

With the ever-growing interest for flexible devices fornext-generation applications, the development of flexiblefield-electron emitters has emerged as an interesting researchtopic. Combination of carbon nanotubes �CNTs�, which areknown for their excellent field-electron emission characteris-tic, with flexible platforms, such as polymer matrixes andplastic substrates, are promising approach to fabricate effi-cient flexible field-electron emitters.1–5 However, the spin,dip, and spray coating of CNT-dispersion solutions on bend-able substrates to form CNT films are not ideal for makingefficient field-electron emitters because neither the verticalalignment nor the pristine surface property of CNTS that arefavorable for field-electron emission is preserved. Literaturesurvey shows that electron emitters fabricated by using CNT-dispersion solutions require a high turn-on applied field andthe maximum emission currents from such emitters are verylimited.

On the contrary, the fabrication of CNT-based flexibleemitters with the preserved vertical alignment of constitutingCNTs is ideal for efficient field-electron source because thelarge field-enhancement factor remains intact. Previously, wereported two different methods to fabricate freestanding flex-ible emitters that consisted of arrays of vertically alignedCNT �VACNT� columns supported by polydimethylsiloxane�PDMS�.6 PDMS has been our choice as a supporting mate-rial due to its favorable electrical, optical, and elastic prop-erties. In the first method, as-grown VACNT-column arrayswere first infiltrated with PDMS prepolymer, and the detach-ment of cured PDMS resulted in freestanding VACNT-arrayemitters. In the second method, as-grown arrays of VACNT

columns were pressed in an up-side-down configurationagainst uncured thin PDMS layer on any flat supports. Sub-sequent curing and detachment of PDMS completed the fab-rication of flexible emitters. Recently, another group used asimilar method to make bendable electron emitters.7 Tsai etal. used a hot-press process for the direct transfer of a forestof VACNTs to a plastic substrate. However, it appears thatthe operation of their emitter is impeded by a poor electricalbackcontact.

In this article, we report the fabrication of large size free-standing field-electron emitters through the transplant ofVACNT-column arrays from silicon substrates to bendablePDMS platforms. The issue of poor cathode contact has beenresolved by carefully etching PDMS at the back side and byattaching a bendable aluminum sheet to the back side of thefabricated emitters. The field-electron emission from thePDMS-supported VACNT-array emitters are tested both inflat and bent-cathode geometries; moreover, after being sub-jected to hundred times of bending cycles.

II. EXPERIMENT

The fabrication sequence of the flexible PDMS-supportedVACNT-array electron emitters is shown schematically inFig. 1. The arrays of VACNT columns that we used to fab-ricate flexible PDMS-supported emitters were grown throughthe combination of a hot-filament chemical vapor deposition�HFCVD� process and prepatterned catalysts, as we reportedelsewhere.8 The 10:1 �volume ratio� mixture of precursor andbase for PDMS �Sylgard 184, Dow-Corning� were mixedthoroughly, and stored in vacuum for 30 min before filmcasting to eliminate any bubbles. The uncured PDMS pre-polymer was spin coated on flat aluminum sheets at 1000rpm for 25 s. The VACNT-column array was laid on thealuminum sheets upside down and presses hard so that the

a�Author to whom correspondence should be addressed; electronic mail:soonil@ajou.ac.kr

C2C5 C2C5J. Vac. Sci. Technol. B 28„2…, Mar/Apr 2010 1071-1023/2010/28„2…/C2C5/4/$30.00 ©2010 American Vacuum Society

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VACNT columns could penetrate the uncured prepolymer allthe way. Next, the PDMS was cured at 120 °C for 2 min.The surface cleanness of aluminum sheets was critical for theuniformity of uncured prepolymer layers and easy detach-ment of PDMS-VACNT films from the aluminum sheets af-ter PDMS curing. The thickness of PDMS support was con-trollable by adjusting the spin-coating conditions �speed andtime� and the PDMS curing temperatures. Subsequently,PDMS-supported VACNT-column emitters were detachedfrom both the silicon substrates and aluminum sheets to re-sult in a freestanding bendable field-electron emitter. We notethat the shape of individual VACNT columns and the arraypattern of VACNT columns were preserved throughout thefabrication process. Finally, the PDMS was etched at theback side by the 3:1 volume-ratio mixture of tetrabutylam-monium fluoride �TBAF� and N-methelpyrrolidone �NMP�.The etching process was carried out with care to exposeCNTs by completely removing PDMS from the backsurfacesof VACNT columns without degrading the overall mechani-cal and elastic characteristics of the PDMS support.6 Figure2�a� shows a SEM image of a PDMS-free top surface of aVACNT column, which is important for field-electron emis-sion, of a flexible 7�7 VACNT-column emitter shown in theinset; the black dots are VACNT columns and the transparentarea corresponds to the PDMS support. A high-resolutionSEM image of CNTs at the top surface of the VACNT col-umn in Fig. 2�b� shows that the VACNTs in the columnmaintain their morphology, and the tips of VACNTs remainclean.

The as-fabricated PDMS-supported flexible VACNT-column cathode was subjected to field-emission measure-ments in a diode configuration at the vacuum of 2�10−6 torr. Initially, the field-electron emission from theflexible PDMS-VACNT cathode was measured in flat-cathode geometry with a planar metal anode that was posi-tioned parallel to the PDMS-VACNT cathode by using aspacer. Next, the field-emission measurements were repeatedin convex-cathode geometry. The deformation of the flexibleemitter was limited to the single-axis bending that resulted inelliptic circumferences, and we were able to estimate thedegree of bending approximately; see Fig. 3. We note thatthe distance a in Fig. 3 corresponds to the length of the longaxis and the diameter b /2 of a wire that props the PDMS-VACNT cathode in the middle to the half of the length of theshort axis of an ellipse.

III. RESULTS AND DISCUSSION

We measured field-electron emission of a flexible emitterthat consisted of 8�8 square array of cylindrical VACNTcolumns with 200 �m diameter and 580 �m height, whichwere supported by 200 �m thick PDMS layer. The 8�8VACNT-column array spanned a square area of 4.0�4.0 mm2 in the middle of the 15�6.5 mm2 rectangularPDMS support, as shown in the plan-view image of the un-deformed VACNT-PDMS emitter in the lower inset in Fig. 3.The VACNT-PDMS emitter is very flexible and can be de-formed manually, as demonstrated in the picture in Fig. 3;see the upper inset. However, the bending of the VACNT-PDMS emitter during the field-emission measurement can be

SiliconVACNTs

(a)

(c)

(d)

500 µm

Hot plate (120 oC)

AluminumsubstratePDMS

Silicon

(b)

Silicon PDMS-supportedVACNT array

TBAF

FIG. 1. �Color online� �a� Sketch of the process used to fabricate a freestand-ing flexible cathode based on a VACNT-column array. First, synthesize aVACNT-column array on a silicon substrate by combining HFCVD andphotolithographic patterning processes: see the SEM image in �b�. Second,lay down the VACNT-column array upside down on a metal sheet coveredwith an uncured spin-coated PDMS prepolymer, then cure PDMS at 120 °Cfor 2 min by using a hot plate �see the photograph in �c��. Third, detach theVACNT-PDMS emitter from both the Si substrate and metal sheet: see thephotograph in �d�. Finally, etch the back side of the freestanding VACNT-PDMS emitter by the mixture of TBAF and NMP.

(a)

(b)

FIG. 2. �Color online� �a� SEM image of the PDMS-free top surface of aVACNT column; inset: illustration of a freestanding PDMS-CNT film; �b�high magnification SEM image of nanotubes top surface showing no trace ofPDMS.

C2C6 Hong et al.: Flexible carbon nanotube-array cathodes: Fabrication and bending effect on field-electron emission C2C6

J. Vac. Sci. Technol. B, Vol. 28, No. 2, Mar/Apr 2010

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quantified in terms of the angle � that the VA-PDMS emittersubtends along an elliptic circumference, which can be de-fined in terms of the long- and short-axis lengths of a and b,when a thin wire of diameter b /2 props cathode in themiddle. As summarized in Fig. 3, the field-electron emissionwas measured at the angles of 90.0°, 53.5°, 40.0°, and 23.5°that correspond to the respective bending angles �=90.0°−�� of 0°, 36.5°, 50.0°, and 66.5°.

Figure 4�a� shows a set of I-V �net current versus biasvoltage� curves that were measured from the VACNT-PDMScathode in flat or bent configurations at various nominalanode-cathode distance and bending angles. The correspond-ing linear Fowler–Nordheim �FN� curves in the inset confirmthat all the measured currents stemmed from the field emis-sion of electrons regardless of cathode configurations. It ap-pears that not only the excellent field-emission characteris-tics of the pristine VACNT-column arrays were preserved inthe flat-cathode geometry �curves I1 and I2� but also that allthe other I-V curves measured under the bent-cathode geom-

etries �curves I3, I4, I5, I6, and I7� showed good filed-electronemission. The bias voltage values to extract the emissioncurrent of 1 �A range from �720 to �2310 V, and thecorresponding nominal magnitude of applied electric field,which is equivalent to the applied bias voltage divided by theanode-cathode distance, ranges from �1.26 to�1.47 V /�m. The net emission current as large as 2 mAwas measured at the configuration of 66.5° bending angleand 570 �m anode-cathode gap. Moreover, the emissioncurrents failed to reach a 1 mA level only when the anode-cathode gap was as large as 1570 �m. These ranges of elec-tric field and net-current values of the flexible PDMS-basedcathode are comparable to those of pristine VACNT-columnarrays on heavily doped silicon.8 This observation provesthat our transplant approach, which relies on the infiltrationof PDMS into VACNT columns to fabricate flexible emitters,has no deteriorating effects on the field-emission propertiesof VACNTs. Furthermore, our flexible PDMS-VACNT cath-ode is very durable so that the I-V curve that was measuredafter going through a few hundred times of bending cycleswas almost identical to that right after the first bending, asshown in Fig. 4�b�: Open and solid circles correspond to themeasurements after a single and repeated �a few hundredtimes� bending, respectively.

We find a couple of interesting trends in the systematicvariation in the I-V curves in Fig. 4�a�. First, higher emissioncurrents were measured when the anode-cathode gap becamesmaller regardless of bending angles: I2�V�� I1�V� andI6�V�� I5�V�� I7�V�. Second, emission currents increasedwith respect to the increase in bending angle when theanode-cathode gap was the same: I7�V�� I1�V�, I6�V�� I3�V�, and I5�V�� I4�V�. However, it is not straightforwardto account for the I-V curve variations in Fig. 4�a�, quantita-tively. Many factors such as uneven anode-cathode gap,position-dependent local electric field at cathode surfaces,and change in series resistance can contribute concomitantlyin response to different bending angles and cathode-anodedistance. Nevertheless, it makes sense to examine the mag-nitude of electric field strength at the position of centralVACNTs, where electric field has the maximum strength, forqualitative discussion. According to the FN field-emissionmodel, filed-electron emission depends exponentially on lo-cal electric field strength, and, therefore, emission currentswill be dominated by central VACNTs’ contribution.

We deduced electric field distributions at the surfaces ofbent cathodes from a series of numerical simulations basedon the two-dimensional COMSOL electrostatic model �version3.4�. The simulations were carried out for the ideal anode-cathode geometries that mimicked the actual configurationsby modeling the bent cathodes in terms of the parameterssummarized in the inset of Fig. 3. Figure 5�a� shows equipo-tential lines corresponding to the bias voltage of 2000 V atthe bending angle of 66.5°. The equipotential lines and elec-tric field distribution over the row of eight VACNT columnsare symmetric, and the maximum field strength appears atthe positions of two central columns as expected. We de-duced the strength of electric fields at the position of column

No a (mm) b/2 (mm) � (o)A 15 0 90.0B 14 1.3 53.5C 13.5 2.6 40.0D 12.5 4.2 23.5

L

Lo

a

Anode

Gap (d)

Cathodeb/2�

FIG. 3. �Color online� Schematic sketch of field-emission test on bendingcathode; a photograph illustrates bendable PDMS-CNT film �upper inset�; aplan-view image of the undeformed SWCNT-PDMS emitter �lower inset�.

0 750 1500 2250 3000

1E-4

1E-3

0.01

0.1

1

I1

I7I4I5I2I3

1234567

Net

curr

ent(

mA)

Voltage (V)

(a)

I6

1 2 3 4

-30

-27

-24

-21

ln(I/

V2 )

[ln(IV

-2)]

1000/V [V-1]

0 750 1500 2250

1E-4

1E-3

0.01

0.1

1

Net

curr

ent(

mA

)

Voltage (V)

(b)

66.5o; 1070 µm

(b)

FIG. 4. �a� I-V curves and corresponding FN plots �inset� of the flexibleCNT-emitter cathode under different bending angle and anode-cathode dis-tance; I1 at 0° and 1570 �m, I2 at 0° and 870 �m, I3 at 36.5° and 570 �m,I4 at 50° and 1070 �m, I5 at 66.5° and 1070 �m, I6 at 66.5° and 570 �m,and I7 at 66.5° and 1570 �m. �b� I-V curves measured at the bending angleof 66.5° and anode-cathode gap of 1070 �m; the first bending �opencircles� and after going through a few hundred times of bending �solidcircles�.

C2C7 Hong et al.: Flexible carbon nanotube-array cathodes: Fabrication and bending effect on field-electron emission C2C7

JVST B - Microelectronics and Nanometer Structures

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A1 and compared their values corresponding to the bias volt-age of 2000 V in Fig. 5�b�. It is very interesting to note thatthe aforementioned two trends in the I-V curve variations areconsistent with the strength of electric fields summarized inFig. 5�b�. A large increase in field strength appears with re-spect to the decrease in the anode-cathode distance at a givenbending angle, and a slight increase in field strength occursin response to the increase in bending angle for a fixedanode-cathode distance. Moreover, we find that the system-atic increase in the emission currents in Fig. 4�a�, I1�V�� I7�V�� I4�V�� I5�V�� I2�V�� I3�V�� I6�V�, is consistentwith the variation in electric field strength corresponding toeach anode-cathode configuration at the identical bias volt-age of 2000 V. The field strength variations at other biasvoltage values are qualitatively similar. Therefore, we canattribute the general trend of the I-V curve variation in Fig.4�a� to the variation in the local electric field strength inresponse to the change in bending angle and anode-cathodedistance.

IV. CONCLUSION

We have presented a facile method for the fabrication offlexible emitters, which consists of the direct transfer ofVACNT-column arrays to the PDMS support and the forma-tion of reliable cathode contact through the combination ofcontrolled etching of PDMS and attachment of thin alumi-num sheets at the back side. This method is so versatile thatany VACNT-column arrays can be transformed into PDMS-supported bendable field-electron emitters. Preliminary field-emission tests have shown that the electron emission fromthe VACNT-PDMS cathode in flat geometry, without anybending, is comparable to that of the pristine VACNT-column arrays, and that efficient field-electron emission ismaintained in convex-cathode geometry with a single-axisbending. Moreover, it has been confirmed that the PDMS-supported VACNT-column emitter is durable against re-peated bending so that no noticeable emission degradationhas been observed after a few hundred times of bending to66.5°. Finally, we carried out a series of simulation to studythe position-dependent distribution of electric field at thecathode surface for various bending angles and anode-cathode distance. Our simulation results showed that the sys-tematic increase in the emission currents can be qualitativelyaccounted for in terms of the variation in the maximum elec-tric field strength at the position corresponding to the centralVACNT columns.

ACKNOWLEDGMENTS

This work was supported by the Korea Science and Engi-neering Foundation �Grant No. R01-2008-000-20689-0� andby the Priority Research Centers Program through the Na-tional Research Foundation of Korea �NRF� funded by theMinistry of Education, Science and Technology �Grant No.2009-0094049�.

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0 10 20 30 40 50 60 70

1.5

2.0

2.5

3.0

3.5

V = 2000 V

I1(0; 1570)

I7(66.5; 1570)

I5(66.5; 1070)

I4(50; 1070)

I2(0; 870)

I3(36.5; 570)

Gap ��m�57087010701570

Loca

lfie

ldA

1(V

/�m

)

Bending angle (o)

I6(66.5; 570)

(b)

Cathode

Anode (2000 V)

(Ground)(a)

A1

FIG. 5. Electric field distributions at the surfaces of bent cathodes from aseries of COMSOL simulation: �a� equipotential lines corresponding to thebias voltage of 2000 V at the bending angle of 66.5° and �b� electric field atthe position of column A1 at the bending angles of 0°, 36.5°, 50°, and 66.5°and the anode-cathode distances of 570, 870, 1070, and 1570 �m.

C2C8 Hong et al.: Flexible carbon nanotube-array cathodes: Fabrication and bending effect on field-electron emission C2C8

J. Vac. Sci. Technol. B, Vol. 28, No. 2, Mar/Apr 2010

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