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Effect of field enhancement on inorganic powderelectroluminescence using short carbon nanotubes
Jin-Young Kim a, Min Jong Bae b,c, Shang Hyeun Park b,*, Taewon Jeong b, Sunjin Song b,Jeonghee Lee b, Intaek Han b, Ji Beom Yoo c, Donggeun Jung a, SeGi Yu d,*
a Department of Physics, BK 21 Physics Research Division, Sungkyunkwan University, Suwon 440-746, Republic of Koreab Electronics Material Lab., SAIT Samsung Electronics, Yongin 446-712, Republic of Koreac SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Koread Department of Physics, Hankuk University of Foreign Studies, Yongin 449-791, Republic of Korea
A R T I C L E I N F O
Article history:
Received 4 June 2010
Accepted 11 August 2011
Available online 7 September 2011
0008-6223/$ - see front matter � 2011 Elsevidoi:10.1016/j.carbon.2011.08.016
* Corresponding authors: Fax: +82 31 280 816E-mail addresses: sh100.park@samsung.c
A B S T R A C T
Inorganic powder electroluminescence (IPEL) devices with the insertion of a carbon nanotube
(CNT) layer were investigated to verify the effect of the increased local field produced by CNTs
on electroluminescence (EL). To increase the field strength effectively, the CNTs were short-
ened using the cryogenic crushing method. IPEL devices with the insertion of a short CNT
layer exhibited an increase in brightness and efficiency with increasing amount of CNTs.
The local field enhancement by CNTs, further enlarged by the triple-junction, could increase
the field strength applied to the phosphor, resulting in improved EL performance. In addition,
short CNTs in an EL device can lead to field enhancement without an unintentional current
flowing into the device.
� 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Carbon nanotubes (CNTs) are considered to be attractive field
emitters, because of their high electrical conductivity and
high aspect ratio with nanometer sizes [1–3]. Studies on their
applications as electronic devices have produced field emis-
sion displays [4] and backlight units for liquid crystal displays
[5]. Recently, another type of field enhancement effect of
CNTs was reported in the field of inorganic powder electrolu-
minescence (IPEL), where a mixture of CNTs and cathodolu-
minescent (CL) phosphor can produce electroluminescence
(EL) along with the local field enhancement of CNTs [6]. On
the other hand, the device using the CL phosphor operated
at relatively high driving voltages (over 250 V) and exhibited
a low intensity of EL compared to conventional IPEL devices,
indicating the importance of field enhancement in IPEL
devices.
er Ltd. All rights reserved
7 (S.H. Park), +82 31 330 4om (S.H. Park), segiyu@h
In general, EL is a phenomenon of light emission of a mate-
rial under the influence of an electric field. The first inorganic
EL was observed from oil suspended ZnS powder under an
alternating current (AC) voltage by Destriau [7]. An EL device
is usually a sandwiched structure of a light-emitting layer be-
tween two electrodes. Among the many EL device structures,
AC-type thick-film IPEL structures have attracted considerable
attention as a potential candidate for illumination and display
applications, due primarily to the advantages in the substrate
flexibility and process, such as the simple fabrication of
large-sized panel, low cost manufacturing and no requirement
of vacuum processing [8–11]. In general, the structure of an IPEL
device is composed of an electrode/dielectric/phosphor/elec-
trode/substrate [12,13], and each component was fabricated
using the screen-printing method [14]. Despite the simple fab-
rication processes in IPEL devices, the actual application of IPEL
devices have been confined to backlight units in cellular
phones due mainly to their insufficient performance, such as
.
566 (S. Yu).ufs.ac.kr (S. Yu).
C A R B O N 5 0 ( 2 0 1 2 ) 1 7 0 – 1 7 4 171
low brightness and efficiency [15,16]. Considerable effort has
been made to improve the properties of IPEL devices – using or-
ganic dye-phosphor composites [17] and adoption of a multi-
emitting-layer structure [18]. However, high brightness with a
simple structure operated at relatively low voltage and low fre-
quency is still required for EL devices as one of future flat panel
displays.
This paper reports the results of the local electric field ef-
fect, which might be enlarged by the existence of the triple-
junction [19,20], in IPEL devices by inserting a separate layer
of CNTs between the dielectric and phosphor layers. To pre-
vent an unintentional current by the CNT networks, the
length of the CNTs was shortened using a cryogenic crushing
method [21]. This study clarifies the brightness improvement
by the local electric field from the CNTs, and proposes a new
structure to construct highly efficient IPEL devices.
2. Experimental
2.1. Preparation of component layers
The top-emission type structure was used for the IPEL devices
(Fig. 1(a)) [22]. Fig. 1 (b) shows a cross-section scanning elec-
tron microscopy (SEM) image. The materials of the compo-
nent layers were prepared as follows. Single wall carbon
nanotubes (SWCNTs, Hanhwa Nanotec, ASP-100F) were
shortened using the cryogenic crushing method for 20 or
30 min [21]. An ethanolic solution of shortened SWCNTs
was dispersed further in dichlorobenzene and treated by ul-
tra-sonication for 2 h. The dielectric paste was prepared by
mixing the BaTiO3 powder (Samsung Fine Chemicals, SBT03)
and a fluoro-resin-based organic binder (ELK Corp., ELPR530)
using ethylene glycol mono-butyl ether acetate as the solvent.
The light-emitting paste was prepared using the same meth-
od of a dielectric paste, except that ZnS:Cu,Cl phosphor
(Sylvania, GG44, encapsulated by alumina) was used instead
of BaTiO3.
Fig. 1 – (a) Schematic diagram of the top-emission typed IPEL
structure with a layer of SWCNTs. (b) SEM image of the
cross-sectional view of an experimentally fabricated IPEL
device.
2.2. Formation of EL devices
Device fabrication was performed using step-by-step coatings
of the layers. First, a dielectric layer was deposited on an alu-
minum-coated glass substrate by screen-printing and dried at
130 �C for 30 min. A shortened SWCNT suspension was then
deposited on the dielectric layer by spin-coating at various
coating-speeds (300, 400, and 500 rpm) and dried at 100 �Cfor 30 min. The phosphor layer was then deposited on the
SWCNT layer by screen-printing and dried at 130 �C for
30 min. Finally, indium tin oxide (ITO) as a top electrode was
deposited on the emitting layer by dc sputtering using an
ITO target (DASOM RMS, xIn2O3–ySnO2).
2.3. Characteristic of EL devices
The characteristics of the devices were measured by applying
a sinusoidal AC voltage under an atmospheric environment.
All the AC readings in this paper refer to the peak-to-peak val-
ues. Combined equipments composed of a conventional func-
tion generator (Agilent, 33250A) and a power amplifier (Trek,
P0610B-K) were used to generate the AC voltages. An oscillo-
scope (Hewlett–Packard 400D) was connected to an IPEL de-
vice to monitor the actual value of the applied high voltage.
The brightness of the IPEL devices was measured using a
luminance colorimeter (Topcon, BM-7). The field emission
characteristics of the shortened SWCNTs were measured in
a vacuum chamber with a diode type configuration under
1 · 10�7 torr, where the CNTs were deposited on the ITO-
coated glass substrate as a cathode plate and the distance be-
tween the cathode and anode (a bare ITO substrate) was
maintained using 400 lm-thick glass spacers.
3. Results and discussion
The untreated SWCNTs (roughly 3 lm long) were shortened to
less than the sub-micrometer range after a cryo-crushing time
of 20 min [21]. The Raman spectral peak at 1325 cm�1 repre-
sents the disorder state of carbon atoms in the graphitic struc-
ture, known as the D-band. According to Fig. 2, the D-band
intensities of the SWCNTs increased with increasing crushing
Fig. 2 – Raman spectra of the untreated, 20 and 30 min-
crushed SWCNTs.
172 C A R B O N 5 0 ( 2 0 1 2 ) 1 7 0 – 1 7 4
time, where the lowest intensity was observed for the non-
treated SWCNTs. The higher D-band intensity of the shortened
SWCNTs originated from a relatively large amount of amor-
phous carbon through the cryo-crushing process [23]. In all
IPEL devices in this study, the crushing time was fixed to
20 min because short SWCNTs for a crushing time of 20 min
could induce effective field enhancement among various
crushing times. The amount of deposited SWCNTs was con-
trolled by the spin coating speed.
The measured luminance of each IPEL device with a layer of
SWCNTs coated at 300, 400, 500 rpm was 192, 182, and 174,
respectively, whereas the luminance for the no CNT was
160 cd/m2 (Fig. 3). This indicates that the luminescence of IPEL
was improved by the presence of SWCNTs, and the high con-
centration of CNT, i.e., obtained using a slow coating speed, re-
sulted in high EL performance. The 300 rpm sample was 20%
brighter than the sample without the SWCNTs [24]. The 20%
improvement in the brightness is still not sufficient to solve
the long-lasting brightness problem for IPEL devices, however,
the adoption of a nanomaterial might hint a new direction for
this problem, whereas the previous researches on EL rather fo-
cused on other issues, i.e., the crystallinity of the phosphor par-
ticle [16,25], the encapsulation of phosphor particle [26],
doubling the emitting layer [9], and incorporation of organic
dye [17]. Considering the recent intensive research activities
in various fields to utilizing nanotechnology [27,28], this ap-
proach, i.e., using a carbon based nanomaterials, at least,
might not lead to a wrong direction.
In general, an electric field in an AC IPEL device is the key
governing factor for the luminance because the intensity of
the luminance can be determined mainly by the intensity of
an electric field [14,29]. ZnS:Cu,Cl phosphor particles have
ZnS–CuxS heterojunctions, where ZnS (CuxS) is a n-type
(p-type) semiconductor. When an electric field of 106–107 V/
m is applied to the phosphor, the electric field induces tunnel-
ing of electrons and holes into ZnS lattice at opposite ends of
each CuxS, then electrons are trapped in the Cl donor site,
while holes are trapped by Cu recombination centers. At the
reversal field, the emitted electrons recombine with the
trapped hole to produce light [14,29]. In the present study,
the SWCNT layer on the dielectric layer, and at the same time
Fig. 3 – Luminance and efficiency as a function of the driving
voltage for the IPEL devices with no SWCNTs and with a
layer of shortened SWCNTs prepared for three coating
speeds (300, 400, and 500 rpm).
beneath the phosphor particle layer, could supply a high local
electric field to the phosphor particles, which might be ex-
plained by a triple-junction type enhancement between the
SWCNT, the dielectric material, and the phosphor [19]. This
field enhancement could not be simply explained by consid-
ering the high geometrical aspect ratio of SWCNTs as ex-
plained in Refs. [19,20].
The existence of the triple-junction between the SWCNT,
the dielectric material, and phosphor particles could be one
of main reasons for this improvement. The abrupt potential
change around SWCNTs, assisted by the work function differ-
ence of the surrounding dielectric and phosphor materials,
could provide a considerable high electric field [20]. The recom-
bination of holes trapped in the Cu recombination centers to-
gether with the increased electrons at the enhanced field at
the reverse voltage will produce more light [14]. Therefore,
the improved EL performance from the IPEL devices with a
layer of SWCNTs might be explained by the enlarged electric
field at the triple-junction.
To clarify the direct relationship of the IPEL performance to
the field enhancement effect asserted from SWCNTs, the field
emission characteristics of the shortened SWCNTs were mea-
sured for three coating speeds. Since the field emission of
SWCNTs on a dielectric layer is difficult due to the insulating
nature of the dielectric layer, the measurements were per-
formed directly on an ITO-coated glass substrate. Although
the magnitude of field enhancement of the shortened SWCNTs
on the dielectric layer might be different from that on the ITO
substrate, the tendency of the enhancing effect to field
enhancement could be matched between CNTs on the dielec-
tric layer and ITO layer. Therefore, the field emission of
SWCNTs on the ITO substrate for the three coating speeds
was measured and the corresponding Fowler–Nordheim (F–N)
plots [30], are shown in Fig. 4. The turn-on electric fields (de-
fined as the electric field at a current density of 10 lA/cm2) were
1.6, 1.8, and 2.1 V/lm, for coating speeds of 300, 400, and
500 rpm, respectively. The dimensionless field enhancement
factors (b) concerning the geometrical contribution of CNT
were 4750, 4500, and 3000, respectively, and the area factors
(a), related to the number of emitting sites were 3.2 · 10�8,
Fig. 4 – Field emission current density as a function of the
applied field for the shortened SWCNTs deposited at
various coating speeds on an ITO substrate. The inset
shows the corresponding Fowler–Nordheim plots.
Table 1 – List of the area factor (a) and field enhancement factor (b) of the shortened SWCNTs on the ITO substrate, and theluminance of the IPEL devices with a layer of shortened SWCNTs.
300 rpm 400 rpm 500 rpm No CNTs
a (10�8 cm2) 3.2 2.6 0.79 –b 4750 4500 3000 –Luminance (cd/m2) 192 182 174 160Luminance increment 20%" 14%" 8.4%" –
Fig. 5 – Luminance and the efficiency as a function of the
driving voltage of the IPEL devices with no SWCNTs and
with a layer of shortened and untreated SWCNTs. The spin
coating speed was set to 300 rpm for all devices. The
difference in the luminance for shortened and untreated
SWCNTs is too small, and it is difficult to distinguish the
two symbols.
C A R B O N 5 0 ( 2 0 1 2 ) 1 7 0 – 1 7 4 173
2.6 · 10�8 and 7.9 · 10�9 cm2, respectively for a 300, 400, and 500
spin speed. Here, the work function of SWCNTs was assumed
to be 5 eV [31]. Among these three samples, the one prepared
at 300 rpm exhibited the highest field emission characteristics,
which is due to the relatively high density of SWCNTs com-
pared to the other speeds. Table 1 summarizes the geometry
factor (b) and area factor (a) of the shortened SWCNTs on the
ITO substrate as well as the luminance of IPEL devices for three
coating speeds. The luminance improvement of IPEL devices
showed a similar tendency of field enhancement obtained
from the ITO substrates. High density of short SWCNTs by
low spin speed could improve the electric field in the IPEL de-
vice because of the triple-junction emission at the interface be-
tween the SWCNT, dielectric materials, and vacuum [19].
In general, the local electric field is increased in proportion
to the field enhancement factor, b, based on the equation,
E ¼ bV: A layer of SWCNTs with a high b value would enhance
the electric field with a help of the triple-junction, facilitating
electron tunneling and acceleration in the phosphor layer. In
addition, the higher a value, representing a larger number of
enhancing sites in field emission, would provide more recom-
bination sites in EL, producing brighter luminescence.
To investigate the length effect of the SWCNTs, three dif-
ferent IPEL devices, i.e., with no SWCNTs, with shortened
SWCNTs, and with untreated long SWCNTs, were prepared
at a fixed spin coating speed of 300 rpm. Their luminescence
characteristics were measured at driving voltages up to 150 V
and 400 Hz. According to Fig. 5, the luminance of the device
with a layer of SWCNTs, whether shortened or untreated,
was higher than that with no SWCNTs.
On the other hand, the EL efficiency of the device with the
untreated SWCNTs was similar to that without SWCNTs, and
lower than the one with the shortened SWCNTs. The effi-
ciency, g, was obtained from the equation, g ¼ p� LV�I (l m/W)
[14], where L is the luminance. The lower efficiency of the un-
treated SWCNT device was caused by the relatively high cur-
rent flow during the EL operation. This might be caused by the
interconnection of long SWCNTs producing a waste current
with no contribution to EL [32]. On the other hand, the device
with the shortened SWCNTs showed similar current flow
with respect to the one without SWCNTs, indicating much
fewer conducting paths in the shortened SWCNT layer.
Therefore, the IPEL device with shortened SWCNTs exhibits
higher efficiency and high luminescence.
4. Conclusions
A series of IPEL devices was fabricated with insertion of a
shortened SWCNT layer between the phosphor and dielectric
layers. The shortened SWCNTs were prepared using the cryo-
genic crushing method to effectively enhance the electric
field but prevent possible conducting paths in the SWCNT
layer. The EL performance improved with increasing amount
of SWCNTs, reflecting the local field enhancement effect by
the shortened CNTs. The evaluation of the EL efficiency for
the IPEL device with long or short SWCNTs highlighted the
importance of the shortened character of the SWCNTs, which
increase the EL efficiency through the high luminance with
low current flow.
Acknowledgements
This work was supported in part by the National Research
Foundation of Korea (NRF 2010-0011894). D. Jung was sup-
ported by grants NRF-2010-0029699 (Priority Research Centers
Program). J.-Y. Kim is a recipient of the Seoul Science Fellow-
ships of the Seoul Metropolitan Government.
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