Physica E 43 (2010) 405–409

Contents lists available at ScienceDirect

Physica E

1386-94

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/physe

Transparent field emitters with light reflector

Seung-Min Lee, Wal-Jun Kim, Yong-Hyup Kim n

School of Mechanical and Aerospace Engineering and the Institute of Advanced Aerospace Technology, Seoul National University, Sillim-dong, Gwanak-gu,

Seoul 151-742, Republic of Korea

a r t i c l e i n f o

Article history:

Received 4 June 2010

Received in revised form

27 July 2010

Accepted 20 August 2010Available online 26 August 2010

77/$ - see front matter & 2010 Elsevier B.V. A

016/j.physe.2010.08.019

esponding author. Tel.: +82 2 880 7385; fax:

ail address: yongkim@snu.ac.kr (Y.-H. Kim).

a b s t r a c t

Carbon nanotube (CNT) network based on single-walled CNT is adopted for transparent field emitters

(TFEs). TFEs play roles of electrode as well as field emitters at the same time. CNT network is prepared

by the spray coating method and CNTs in the network are vertically aligned for field emitters by the

triboelectricity method [1]. It shows a low threshold field of 1.7 V/mm corresponding to the current

density of 10 mA/cm2 and stable lifetime performance. A diode-type field emission device is prepared by

using TFEs. Transparency of TFE cathode allows an inverted design [2] of a diode-type field emission

device, where light comes out not from an anode side as in a conventional design but from a cathode

side. Therefore, light reflector can be installed on the anode side below a phosphor layer replacing a

transparent electrode such as indium tin oxide (ITO), which enhances illuminance of the device. The

diode-type field emission device composed of the present TFEs and an aluminum light reflector shows

35% higher efficiency of illuminance than the conventional device. Therefore, TFEs with light reflector

have a strong potential in field emission devices.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Since CNT was discovered in 1991 [3], it has become anattractive material for field emitter due to its superior propertiesin electrical, geometrical and chemical aspects to existingemitters such as molybdenum tip. CNTs can be directly synthe-sized [4,5] or indirectly attached [1,6–11] on a substrate to applythem to a cathode of the field emission device. The directsynthesis of the CNT emitter is generally based on a chemicalvapor deposition method. CNTs can be synthesized on a substrateby using metal catalysts and carbonaceous gases at hightemperature. Though the method usually provides strong adhe-sion between the synthesized CNTs and the substrate, it is notappropriate for practical applications due to high temperatureabove 500 1C during the process, which severely limits theselection of the substrate material. On the other hand the indirectattachment method including screen printing, dip coating, spraycoating, spin coating and vacuum filtration provides CNT net-works on a substrate in the form of a thin film. It is a simpleprocess performed at relatively low temperature, and therefore,the selection of the substrate is more flexible. However, themethod usually suffers from weak adhesion between CNTs andthe substrate or other problems such as outgassing.

Screen printing with CNT paste is one of the indirect methodsand has been extensively investigated because of its suitability for

ll rights reserved.

+82 2 880 1728.

mass production of large scale applications and good adhesionforce. However, the paste usually includes organic binders, whichgradually degrade field emission performance due to outgassing.Moreover, it requires additional activation process such as a stickytape treatment in order to make CNTs vertically aligned [12,13].The tape process frequently results in secondary contaminationdue to the organic residues of the tape.

In the present study, transparent field emitters are fabricated bythe spray coating method and the triboelectricity method. CNTnetwork is formed on a glass substrate by spray coating of CNTcolloidal solution and then surface of the network is activated for fieldemitters by using the triboelectricity method. CNT spray coating isadvantageous for mass production due to its simplicity andreproducibility. The triboelectricity method is effective for activatingCNTs as TFEs without leaving organic residues because it utilizesfrictional electrostatic force. As a result, the activated CNT networkcould work for electrode as well as field emitters at the same time.

2. Experimental

Single-walled CNT (ASP-100F, Hanwha nanotech Co. Ltd.,Korea) is utilized in the present study, which is synthesized byan arc-discharge method. The single-walled CNT has the diameterof 1–1.2 nm and the length of 5–20 mm. In order to eliminateimpurities such as amorphous carbons, carbonaceous particlesand metal catalysts included in the CNTs, ultrasonication in nitricacid is performed for 30 min. The acid treatment not only removes

S.-M. Lee et al. / Physica E 43 (2010) 405–409406

impurities but also forms carboxyl and hydroxyl groups on thesurface of CNTs, which helps dispersion in the organic solvent.Then well-dispersed CNT colloidal solution is prepared byultrasonication in 1,2-DCB (0.1 mg/ml) for 20 h.

Soda lime glass (2.5�7.5 cm2) is separately prepared as asubstrate. In order to remove organic matters on the surfaceof the substrate, acetone cleaning for 5 min and piranha cleaning(1:4, v/v, H2O2:H2SO4) for 10 min are performed in sequence.

Spray coating is carried out to form the CNT network on thesubstrate. Inert gas of pure nitrogen with the pressure of 1 kg/cm2

is used for the spray coating system to form a homogeneous CNTnetwork because CNTs are easily agglomerated in a colloidalsolution due to dust and moisture included in air. The substrate isheated up to 200 1C by a hot plate during the spray coatingprocess to increase the coating efficiency. Several specimens areprepared by adjusting the sprayed amount of CNT colloidalsolution of 0.5, 1, 3, 5 and 7 ml.

Fig. 1. Structure schematic of conventional diode-type geometry (a) and inverted

geometry with Al reflector (b). Visible light (IA+ IC) generated from phosphor is

split according to propagation direction in a conventional diode-type structure. In

contrast, inverted geometry with Al reflector makes IA and IC propagate to the

same direction. As a result, light intensity (Iaccumulated) of transparent FEs can be

higher than light intensity (IA) of existing frontal light emitting displays through

high transmittance (a40.5).

Fig. 2. Electrical and optical characteristics of homogeneous CNT networks. (a) Sheet r

networks on glass substrate. (b) Transmittance plot according as sprayed amount. Tr

transmittance difference on symbol mark of Seoul National University in 3 ml case (75

In order to make the CNT network effective for a field emitter,it is advantageous to vertically align CNTs by an activationprocess. Various methods can be used for the activation of CNTsand the taping method is one of the frequently used treatments.However, the taping method usually leaves organic residues aftertreatment, which gradually degrades the performance of the fieldemission device due to outgassing. Moreover, the taping methodusually results in excessive loss of CNTs in the CNT networkprepared by the spray coating method. In the present study, thetriboelectricity method is adopted for the activation treatment toavoid the above mentioned problems.

The diode-type device with TFEs cathode and aluminum (Al)light reflector is prepared to demonstrate the field emissioncharacteristics. A diode-type device with conventional geometry isprepared separately to compare the performance with the presentdevice. The present device is composed of CNT/glass structure for acathode and phosphor/Al/glass structure for an anode while theconventional device is composed of CNT/glass structure for acathode and phosphor/ITO/glass structure for an anode. Al reflectorfabricated by thermal evaporation has the thickness of 300 nm. Thelight comes out from the cathode side in the present device, while itcomes out from the anode side in the conventional device. Highvoltage supplier (Keithley 248), multimeter (Keithley 2000), illumi-nance meter (TES Digital Illuminance Meter, TES-1330A), andLabview program are utilized to measure the field emissionperformance of the devices. The field emission test is performedunder a high vacuum condition of low 10�6 Torr. The distancebetween the anode and the cathode is 1000 mm and the emissionarea is 1 cm2. The illuminance meter is located 10 cm above the fieldemission device under dark room condition.

3. Results and discussion

Fig. 1 shows schematic drawings of diode-type field emissiondevices with a conventional design and the present designcomposed of the TFEs and a light reflector. As shown in thefigure, only half of the light emitted from phosphors can beutilized in the conventional device with opaque cathode. How-ever, the present device theoretically allows utilizing whole lightemitted from phosphors because light emitted to the backwarddirection is reflected to the forward direction by the reflector.Accumulated light intensity (Iacc) with the light reflector can beevaluated by a simple equation below if it is assumed that lightlosses due to a glass substrate and a phosphor layer are negligibleand reflexibility of the light reflector is perfect.

Iacc ¼ 2IC ¼ 2aIA

esistance plot according to the sprayed amount. Inset indicates homogeneous CNT

ansmittance is inversely proportional to sprayed amount. Inset indicates visible

%) of sprayed amount.

S.-M. Lee et al. / Physica E 43 (2010) 405–409 407

where I is light intensity, a is transmittance of the TFEs, andsubscripts A and C indicate anode and cathode directions,respectively. It is expected that the accumulated light intensity(Iacc) of the present device will be higher than light intensity (IA)of the conventional device if a is higher than 0.5. Since it is easy tofabricate a CNT network with transmittance larger than 0.5 andreasonable electrical resistance, the TFEs with light reflector has astrong potential for field emission devices in terms of emissionperformance as well as power saving.

Fig. 2 shows sheet resistance and transmittance of the CNTnetwork on the glass substrate with respect to the amount of CNTcolloidal solution sprayed. As shown in the figure, sheet resistancevaries from 3669 to 132 O/&. CNTs work as conducting sticks withrandom distribution in the network because they have high aspectratios without preferential direction [14]. If the sprayed amountincreases, the number of conducting path increases, and therefore,sheet resistance decreases. The inset of Fig. 2(a) shows ahomogeneous CNT network coated on the substrate. Transmittanceof the CNT network varies from 93% to 50% with almost a linear

Fig. 3. Description of triboelectricity method. (a) Schematic and process sequence of t

individual CNTs and makes CNTs at surface of network vertically aligned. (b) Working f

the other forces at d¼12 A, is the effective working force on vertical alignment of C

triboelectricity method. Insets indicate high magnified images about vertically aligned

relation as shown in the figure. When the spayed amount increases,scattering probability of electromagnetic wave increases, andtherefore, transmittance gradually decreases. As shown in the insetof Fig. 2(b), the glass substrate after CNT coating (right part) looksdarker than the one before CNT coating (left part).

As discussed above, it is required that the transmittance of theCNT network should be higher than 50% with reasonable sheetresistance to achieve a more efficient field emission devicethan the conventional one. The CNT network prepared with thesprayed amount of 3 ml is chosen in the present study becausethe corresponding transmittance (75%) and the sheet resistance(347 O/&) satisfy the requirements.

Since the electrostatic force induced by the triboelectricity isstronger than the Van der Waals force between CNT and substrate,CNTs stuck on the substrate can be pulled up and vertically alignedby using the triboelectricity method. The activation process based onthe triboelectricity method is described in Fig. 3(a). A polyethylene(PE) film with the thickness of 13 mm is covered over the CNTnetwork and rubbed with a cotton ball. Then the frictional

riboelectricity method. Electrostatic force induced by triboelectricity pulls up the

orces on CNT during triboelectricity method. Electrostatic force, over 2 times than

NTs. (c) SEM images (tilt view) of CNT network on glass substrate before/after

CNTs.

Fig. 5. Comparison plot of illuminance between inverted geometry with Al

reflector (red circle) and conventional diode-type geometry without Al reflector

(black rectangular). Compared with conventional diode-type geometry, illumi-

nance of inverted geometry with Al reflector increased at whole input power. It

indicates that the reflector can improve device efficiency and power saving.

Maximum improvement (about 35%) is at 0.44 W of input power. Illuminance of

the recommended office lamp, based on American National Standards Institute, is

320 lux. (For interpretation of the references to color in this figure legend, the

reader is referred to the web version of this article.)

S.-M. Lee et al. / Physica E 43 (2010) 405–409408

electrification induces positive charges and negative charges on thecotton ball and the PE film, respectively. The sign of the inducedcharge is determined by the difference in the electron affinitybetween two contact materials. When the cotton ball is removed,positive charges are naturally induced on the CNT network due tonegative charges existing on PE. Then the attractive electrostaticforce is generated between the PE film and the CNT network.Therefore, when the PE film is separated from the CNT network, theelectrostatic force pulls up the CNTs and makes them verticallyaligned.

When we apply the triboelectricity treatment to the CNTnetwork, Van der Waals force as well as electrostatic force existsbetween the CNTs and the PE, while there exists only Van derWaals force between the CNTs and the glass substrate as shown inFig. 3(b). Effectiveness of the triboelectricity method can beverified by evaluating the magnitude of these forces. Thefollowing is a closed form equation of Van der Waals force forthe contact of cylinder–planar surface or cylinder–cylinder[15,16].

F=length¼ ðA12d0:5eff Þ=ð16Z2

OÞ

where A12 is the Hamaker constant for materials ‘‘1’’ and ‘‘2’’under vacuum condition, deff is the diameter (d) of the cylinder forcylinder–planar surface contact or the effective diameter of twocylinders (d1d2/(d1+d2)) for cylinder–cylinder contact and ZO isthe distance between two contact materials. It is assumed that d

(¼d1¼d2) and ZO are 12 A (the diameter of individual single-walled CNT) and 4 A, respectively. If the Hamaker constant of CNTis assumed to be the same as that of graphite, Aglass-CNT, ACNT-PE

and ACNT-CNT are 2.12�10�19, 1.45�10�19 and 2.75�10�19 J,respectively [17]. Then Van der Waals forces of Fglass-CNT/length,FCNT-PE/length and FCNT-CNT/length are evaluated as 2.87�10�6,1.96�10�6 and 2.63�10�6 N/m, respectively. It is confirmedthat only Van der Waals force between CNT and PE is insufficientto make CNT aligned vertically.

Charges per unit length induced on CNT by triboelectricity areevaluated as 1.274�10�17 C/m [1], and then electrostatic forceper unit length (based on F¼keq1q2/r2) is 9.12�10�6 N/m.Therefore, the combined force of electrostatic force and Van derWaals force between CNT and PE is larger than Van der Waalsforce between CNT and glass or CNT and CNT, which allows CNTsto be pulled up and aligned vertically by using the triboelectricitymethod. It is confirmed that the triboelectricity method iseffective to activate CNTs for field emitters. The scanning electronmicroscopy (SEM) image of vertically aligned CNTs after thetriboelectricity treatment is shown in Fig. 3(c).

As shown in Fig. 4(a), the present TFEs show a turn-on field(corresponding to 10 mA/cm2 of current density) of 1.7 V/mm and

Fig. 4. Field emission characteristics of transparent FEs in 3 ml case of sprayed amoun

and current density at 2.6 V/mm is 0.42 mA/cm2. (b) Lifetime test for 10 h. Current den

a current density of 0.31 mA/cm2 at 2.9 V/mm. Lifetime test for theTFEs is performed and the results are plotted in Fig. 4(b).Normalized current density converges to 0.43 after 10 hours,which corresponds to the current density of 0.63 mA/cm2. TheTFEs show field emission performances enough for displayapplication and stable lifetime performance.

Illuminances of the present device (Iacc) and the conventionaldevice (IA) are measured to demonstrate the efficiency of thepresent device. As shown in Fig. 5, Iacc is higher than IA over thewhole range of input power. Illuminance of the present device isenhanced by 35% at the input power of 0.44 W. These results aresimilar to that of Jang et al. [18]. Though the enhancement islower than 50% of the theoretical value due to several lossesoriginating from light absorption and heat conversion, theilluminance of the present device is much larger than theconventional device. Aluminum (Al) layer was first introducedas a light reflector for field emission device by Fran and Tseng[19]. Since field emitted electrons should pass through the Allayer in their design, and then activate phosphor, the devicerequires electric potential higher than 9 kV. However, the presentdevice requires the same potential level as the conventional

t. (a) Current density plot according to input electric field. Turn-on field is 2 V/mm

sity is normalized by initial current density (1.4 mA/cm2).

S.-M. Lee et al. / Physica E 43 (2010) 405–409 409

device because the light reflector is located below the phosphorlayer. Therefore, the present design with light reflector is muchmore effective for the field emission device.

4. Conclusion

TFEs are fabricated by the spray coating method and thetriboelectricity method. CNT network is fabricated on a glasssubstrate by spray coating and then the surface of the network isactivated for field emitter by using triboelectricity method. Spraycoating results in a CNT network free from organic binder andadvantageous for mass production. The triboelectricity methodactivates CNTs for field emitters without leaving organic residuesas compared to the taping method because it is an activationprocess based on frictional electrification and electrostatic force.Transparency of the present TFEs cathode allows adopting a lightreflector, which enhances the illuminance of the device muchhigher than the conventional device.

The present device composed of TFEs and light reflector showsfield emission performance enough for display application andstable lifetime, and therefore, has a strong potential in fieldemission displays.

Acknowledgments

This research was supported by the Korea Science andEngineering Foundation (KOSEF) grant funded by the Koreagovernment (MOST) (Nos. 2008-2001901 and 2009-0078219),Basic Science Research Program through the National ResearchFoundation of Korea (NRF) funded by the Ministry of Education,Science and Technology (No. 2009-0083512), the NationalResearch Foundation of Korea (NRF) grant funded by the Koreagovernment (MEST) (Nos. 2009-0078659 and R15-2003-032), thePioneer R&D Program for Converging Technology and Basic

Research Promotion Fund (Grant number M10711270001-08M1127-00110) and the second stage of the Brain Korea 21Project in 2009.

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