presentation_file_52962682-403c-47f2-968f-5e62ac102812

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Abstract

The CURF Technology

Featurehighlights an

emerging technology

developed at Clemson

Universitywhich is

currently available for

licensing. See inside for a

introduction to this

technology and contact

CURF for more

information.

High Density Atmospheric Plasma

Jet Devices for Biomedical and

Electronic Surface Modifications

cont actcur f @cle ms on.e du w ww .c lems on.ed u/c ur f 864 .656.5157


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For more information: [email protected] 864.656.4237 www.clemson.edu/curf

High Density Atmospheric Plasma Jet Devices for Biomedical and Electronic

Surface Modifications

Description:

Thin films have a wide variety of use in engineering applications such as microelectronics, aerospace

applications, biomedical applications among others. Current deposition techniques include sputtering,

physical vapor deposition, chemical vapor deposition and electrochemical deposition. These deposition

techniques are still seeking ways to improve the behavior in the quality and uniformity of the film,

growth control and the geometry of the growth. Biomedical thin film coatings have to be free of defects

and need a high level of process control for uniformity and adhesion.

This invention is aimed to address this need through utilization of a high

density plasma emission by plasma jet-to-jet interaction in a honeycombstructure plasma jet array device. This plasma concentration behavior by

jet-to-jet interaction enables diverse applications with a simple

configuration and allows new material surface possibilities in an cost-

effective and safe manner.

Applications

Decontamination/Sterilization systems Energy storage related material surface applications Biomedical device surface modification Plasma etching

Benefits:

Ability to achieve uniform coating Easy and safe cold plasma implementation Cost-effective surface modification

Inventors: Sung-O Kim, Jae-Young Kim

Protection Status: A patent application has been filed

Licensing Status: This technology is available for licensing

CURF Reference: 2010-064


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IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology23 (2012) 485606 (8pp) doi:10.1088/0957-4484/23/48/485606

Low-temperature growth of

multiple-stack high-density ZnOnanoflowers/nanorods on plasticsubstrates

Do Yeob Kim1, Jae Young Kim1, Hyuk Chang2, Min Su Kim3,

Jae-Young Leem3, John Ballato4 and Sung-O Kim1

1 Holcombe Department of Electrical and Computer Engineering, Center for Optical Materials Scienceand Engineering Technologies (COMSET), Clemson University, Clemson, SC 29634, USA2 Samsung Advanced Institute of Technology, Samsung Electronics, Yongin 446-712, Korea3 Department of Nano Systems Engineering, Center for Nano Manufacturing, Inje University, Gimhae

621-749, Korea4 School of Materials Science and Engineering, Center for Optical Materials Science and Engineering

Technologies (COMSET), Clemson University, Clemson, SC 29634, USA

E-mail:[email protected]

Received 13 August 2012, in final form 17 September 2012

Published 6 November 2012

Online atstacks.iop.org/Nano/23/485606

Abstract

Reported here is the low-temperature growth of multiple-stack high-density ZnO

nanoflower/nanorod structures on polyethylene naphthalate (PEN) substrates derived from the

surface modification of ZnO seed layers using an atmospheric-pressure plasma jet (APPJ)

treatment. The plasma treatment could provide several advantages to the growth of

multiple-stack ZnO nanoflower/nanorod structures: (i) the surface wettability of the seed

layers changes from hydrophobic to hydrophilic, resulting in higher surface energies for the

growth of high-density ZnO nanoflowers, (ii) the nucleation sites increase due to the increased

surface roughness caused by the plasma etching, and (iii) there is no thermal damage to the

plastic substrate from the plasma treatment due to its low-temperature weakly ionized

discharge. It was also confirmed that multiple stacks of ZnO nanoflowers were obtained

without degradation of the crystal quality or modification to the crystal shape or phase. The

ZnO nanoflower/nanorod structures grew by lengths up to 4 m due to an increased surface

roughness of 10% and surface energy 5.5 times that of the seed layers. As shown, the APPJ is

a very good method to obtain high-density ZnO nanostructures on plastic substrates below150 C, as is critical for flexible electronics.

(Some figures may appear in colour only in the online journal)

1. Introduction

Zinc oxide (ZnO) is regarded as one of the most important

semiconductor materials due to its useful electro-optic and

electro-mechanical properties, including piezoelectricity, UV

luminescence, large exciton binding energy, high electron

mobility, and chemical/thermal stability. Since both theshape and size of nanostructures can influence the resultant

properties, there have been numerous reports focused on

engineered nanostructures including nanorods (or nanowires),

nanotubes, nanosheets, nanoflowers, and nanospheres grown

by changing growth conditions [111].

Among the diverse range of ZnO nanostructures,

nanoflowers have the particular advantages of high surface

area to volume ratio and a short conduction path for electronsto transport. These features are highly preferred for bio-


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Nanotechnology23 (2012) 485606 D Y Kimet al

device, display, sensor, and solar cell applications [1216].

There are two principal synthetic approaches that yield ZnO

nanoflowers: a vapor-phase method and a solution-phase

method. The vapor-phase method produces high-quality ZnO

nanoflowers under well controlled high-vacuum conditions.

However, it is not an appropriate technology for flexible

electronics on plastic substrates because of the highgrowth temperatures (400600 C) [17, 18]. Conversely,

the solution-phase method has been used widely as an

alternative technique because of the simpler equipment, more

environmentally friendly chemicals, larger-capacity growth

vessels, and lower cost. More importantly, the solution-phase

method can be performed at low temperatures (100200 C),

permitting the use of a wide variety of substrate materials.

To date, ZnO nanoflowers have mostly been synthesized in

the form of nanopowders without a substrate [1922]. ZnO

nanoflowers grown on substrates have only been single stacks

due to the low surface energy and low roughness of the various

template layers, which conventionally are grown on substrates

before the growth of the ZnO nanostructures [2325].There are no reports to the authors knowledge of the

growth of multiple stacks of ZnO nanoflowers on plastic

substrates because of the general lack of a method that

simultaneously can provide a large enough surface energy

and appropriate roughness of the template layers. Therefore,

it is necessary to develop an effective growth method to

fabricate multiple-stack high-density ZnO nanoflowers on

plastic substrates that permit high electron transport capacity

with a high dielectric constant while maintaining a high

surface area to volume ratio.

In this work, we report the first synthesis of multiple-

stack high-density ZnO nanoflowers/nanorods on plasticsubstrates using a hydrothermal method at temperatures below

150 C. The multiple-stack high-density ZnO nanoflowers

were obtained using an atmospheric-pressure plasma jet

(APPJ) treatment of the seed layers. Several advantages are

expected from the plasma treatment on the plastic substrates:

(i) the surface wettability of the seed layers changes from

hydrophobic to hydrophilic, resulting in higher surface

energies for the growth of high-density ZnO nanoflowers,

(ii) the nucleation sites increase due to the increased surface

roughness caused by the plasma etching, and (iii) there

is no thermal damage to the plastic substrate from the

plasma treatment due to its low-temperature weakly ionized

discharge.

2. Experimental section

2.1. Synthesis of the ZnO seed layers

Polyethylene naphthalate (PEN, DuPont Teijin Films)

substrates were cleaned ultrasonically in a mixed solution

of isopropanol, ethanol, and deionized (DI) water (volume

ratios of 1:1:1) for 10 min. ZnO seed layers then were

deposited on PEN substrates using a solgel spin-coating

method. Specifically, the solgel solution was prepared by

dissolving zinc acetate dihydrate [Zn(CH3COO)22H2O] in amixture of 2-methoxyethanol and monoethanolamine (MEA),

used as a solvent and stabilizer, respectively. The molar ratio

of the MEA to zinc acetate dihydrate was held constant

at 1.0; the concentration of zinc acetate was 0.5 M. The

resultant solution was stirred at 60 C for 2 h to yield a clear

and homogeneous solution, and then it was aged at room

temperature for 24 h. The solution was drop-cast onto PEN

substrates and spin-coated in two steps. The first and secondcoating steps were at 1000 rpm for 10 s and then at 3000 rpm

for 20 s, respectively. After spin-coating, the ZnO seed layers

were heated at 100 C for 20 min in order to evaporate the

solvent and remove the organic residue. The ZnO seed layers

then were treated with the APPJ array for 30 s. This procedure

from the coating to the plasma treatment was repeated four

times.

2.2. Atmospheric pressure plasma system

In order to treat a wide area with the plasma, a plasma jet

array device comprised of seven quartz tubes was fabricated.

The complete APPJ system is described schematically in

figure 1(a). The array was formed from a central quartz

tube surrounded by six tubes, each having an inner diameter

of 1 mm and an outer diameter of 2 mm such that the

center-to-center distance between two adjacent quartz tubes

was 2.3 mm. Copper tape, 6 mm in width, was used as a

powered electrode and was wrapped around each quartz tube

10 mm from the end of the tube. Carbon tape was employed

as the seven tubes were combined by the powered electrode

with the copper tape. An indium tin oxide- (ITO-) coated

glass plate of 0.8 mm thickness was placed 10 mm from

the end of the quartz tubes with the glass side facing the

plasma jets and served as a ground electrode. High purity(99.997%) helium gas was used as the discharge gas, with a

flow of 5 standard liters per minute (slm). When a sinusoidal

voltage with peak value of 6 kV and frequency of 32 kHz

was applied to the powered electrode, the plasma plume

between the plasma jet array device and the glass side of

the ITO-coated glass exhibited seven well collimated plasma

plumes, as is shown in figure 1(b). The plasma plumes were

well aligned and parallel to each other under these conditions.

The ZnO seed layers grown on the PEN substrate were

then positioned onto the ITO-coated glass in preparation for

subsequent plasma treatments. When these seven collimated

plasma plumes impinged upon the surface of the ZnO seed

layers, the plasma plumes formed a uniform plasma layer. The

effective area treated by the plasma plumes in this experiment

was about 1.3 1.3 cm2.

2.3. Synthesis of the ZnO nanoflowers/nanorods

A hydrothermal method was used to grow the ZnO

nanoflower/nanorod structures on the as-prepared and plasma-

treated ZnO seed layers. The samples were transferred into

a Teflon-lined autoclave (125 ml) that contained an aqueous

solution of 0.05 M zinc nitrate hexahydrate [Zn(NO3)26H2O]

and 0.05 M hexamethylenetetramine [C6H12N4]. The samples

were held in position by a holding structure inside theTeflon-lined autoclave at a height of 2.2 cm from the bottom.


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Figure 1. (a) Schematic diagram of the atmospheric pressureplasma jet (APPJ) system. (b) Photograph of seven well collimatedplasma plumes between the plasma array and the polyethylenenaphthalate (PEN) plastic substrate.

The sample and the holding structure were completely

submerged in the aqueous solution. The growth temperature

was held constant at 150 C for 10 h. After the reaction, the

sample was rinsed thoroughly with DI water and dried with

flowing nitrogen gas in order to remove residual salts and

organic material.

2.4. Characterization

The optical emission spectrum of the APPJ array was

monitored using a fiber optic spectrometer (Ocean Optics,

USB-4000 UVvis) in order to identify the reactive speciesgenerated by the helium plasma plumes in the ambient

air. The wettability of the ZnO seed layers was measured

using a contact angle goniometer (KSV, CAM 200),

whereas the surface morphology was observed by atomic

force microscopy (AFM; Digital Instruments, Dimension

3100) in tapping mode. Field-emission scanning electron

microscopy (FE-SEM; Hitachi, S-4800) and transmission

electron microscopy (TEM; Hitachi, H-9500) were also used

to characterize the morphology and crystal size of ZnO. Prior

to the TEM measurement, ZnO nanoflowers/nanorods were

dispersed ultrasonically from the PEN substrate in acetone

for 30 min, and then a few drops of this mixture (ZnO in

acetone) were placed on the copper TEM grids and driedfor subsequent observation [26]. The crystallinity and crystal

Figure 2. Optical emission spectrum of the APPJ array, which wasmonitored using a fiber optic spectrometer.

phase of the ZnO nanoflowers/nanorods were analyzed by

x-ray diffraction (XRD; Rigaku, ULTIMA IV diffractometer)

using Cu K radiation ( = 1.54 A). The optical properties

of the ZnO nanoflower/nanorod structures were investigated

using Raman spectroscopy (Thermo Scientific, Almega XR)

in a backscattering geometry using the 488 nm emission line

as an excitation source.

3. Results and discussion

Figure 2 provides the optical emission spectrum of the

helium plasma plumes in ambient air. Various excited species

including N2, N+2, He, and O were observed. The clearpresence of nitrogen and oxygen species in the emission

spectrum indicates that many gaseous species from the air

participate in the plasma processes, even though the plasma

jets are produced only from pure helium gas. The reactive

oxygen species in the plasma volume can effectively change

surface properties of materials with which it comes into

contact. In order to investigate the interfacial properties of

the ZnO following the plasma treatment, the wettability of

the ZnO seed layers was characterized by contact angle

measurements. The water contact angle was found to decrease

from 103.0 to 40.5, as shown in figure 3, which indicates

that the surface wettability of the ZnO seed layers waschanged from a hydrophobic to hydrophilic nature by the

plasma. Surface energies of the ZnO seed layers were obtained

using the GirifalcoGoodFowkesYoung equation [27]

sv=lv(1 + cos )

2

4 . (1)

Here, sv and lv are the interfacial surface energies of the

solidvapor and liquidvapor interfaces, respectively. For lva surface energy of 72.5 mJ m2 was used for the DI water,

whereas for sv the measured value of contact angle from

the ZnO seed layers was used [28]. The calculated values of

the surface energy are summarized in table 1, and reveal that

the surface energy of the ZnO seed layers is increased 5.5-foldby the plasma treatment.


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Figure 3. Contact angle measurement of the (a) as-prepared ZnOseed layers and (b) plasma-treated ZnO seed layers. The insets showa photograph of a water droplet on each seed layer.

The AFM and FE-SEM images of the as-prepared and

plasma-treated ZnO seed layers are shown in figure 4. It wasfound that the plasma treatment results in a 10% increase of

Table 1. Contact angle and surface energy of the ZnO seed layers.

SampleContactangle (deg)

Surfaceenergy (mJ m2)

As-prepared ZnOseed layers

103.0 0.84 10.89

Plasma-treated ZnO

seed layers

40.5 0.24 56.17

surface roughness of the seed layers from 2.783 to 3.063 nm,

which can be explained by the known etching effects of

plasma [29].

Figure 5 provides FE-SEM images of the ZnO

nanoflowers/nanorods grown on the as-prepared ZnO seed

layers as well as the plasma-treated ZnO seed layers. ZnO

nanoflowers/nanorods consist of a two-layered structure with

nanorod arrays on the bottom layer (red rectangles) and

nanoflower arrays on the top layer (blue rectangles). The

individual nanoflower structure is composed of a numberof hexagonal nanorods, which grow radially in many

directions. It also was found that the crystal shape of the

ZnO nanoflowers/nanorods is barely affected by the plasma

treatment.

Figures6(a)(d) show cross-sectional FE-SEM images of

the ZnO nanorod arrays grown on the seed layers. The typical

length and diameter of the individual nanorods range from 500

to 1000 nm and 50 to 100 nm, respectively. The probability

distributions for the length and diameter of the nanorods

are shown in figures6(e) and (f). The dominant length and

diameter of the plasma-treated sample were smaller than those

of the as-prepared sample. The size of the ZnO nanorods

was non-uniform due to the growth of some nanorods beinghindered by their neighbors, which halted the subsequent

Figure 4. AFM images of the (a) as-prepared ZnO seed layers with RMS roughness of 2.783 nm and (b) plasma-treated ZnO seed layers

with RMS roughness of 3.063 nm. Top-view FE-SEM images of the (c) as-prepared ZnO seed layers and (d) plasma-treated ZnO seedlayers.


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Figure 5. Top-view FE-SEM images of the ZnO nanoflower/nanorod structures grown on the (a) as-prepared ZnO seed layers and (b)plasma-treated ZnO seed layers. The insets show high-magnification images of the nanorods on the bottom layer (red rectangles) andnanoflowers on the top layer (blue rectangles).

Figure 6. Cross-sectional FE-SEM images of ZnO nanoflowers/nanorods grown on the (a), (c) as-prepared ZnO seed layers and (b), (d)plasma-treated ZnO seed layers and the histograms of the (e) length and (f) diameter distribution of the nanorods.

growth. It is noticeable that the height and density of the

ZnO nanoflowers/nanorods were significantly increased by

the plasma treatment of the ZnO seed layers. As is shown

in figures6(c) and (d), multiple stacks of nanoflowers were

grown continuously on the nanorods in the plasma-treated

sample, whereas only one stack of nanoflowers was grown onthe nanorods in the as-prepared sample.

The increase in roughness of the seed layers following

the plasma treatment induces the growth of ZnO nanorods

with rougher surface morphology. This implies that there are

a number of nanorods whose height is greater than those of

adjacent nanorods. For the growth of the ZnO nanoflowers

following the growth of the nanorods, nucleation sites areformed on the surface of the ZnO nanorods with greater height


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Figure 7. XRD patterns of the ZnO nanoflower/nanorod structuresgrown on the (a) as-prepared ZnO seed layers and (b)plasma-treated ZnO seed layers. The peaks denoted with an asterisk

are attributed to the PEN substrates.

because of the radial growth preference of the nanorods,

which is less spatially hindered. After the formation of the

nucleation sites, many ZnO nanorods grow radially, which

finally results in the ZnO nanoflower structure. Furthermore,

the increased surface energy of the plasma-treated ZnO seed

layers results in multiple stacks of nanoflowers, because the

nucleation probability is increased in order to reduce the entire

system energy.

Figure 7 provides the XRD pattern associated with

the ZnO nanoflower/nanorod structures grown on the as-

prepared and plasma-treated ZnO seed layers. Various ZnO

diffraction peaks were observed, including those at 31.8

,34.46, 36.28, 47.58, 62.88, 68.02, and 69.2 two-theta,

which correspond to the following ZnO crystallographic

reflections: (100), (002), (101), (102), (103), (112), and (201),

respectively. Except for the peaks from the PEN substrate,

which are marked with an asterisk in figure7, all of the peaks

in the obtained spectrum are well indexed to hexagonal ZnO

phase (JCPDS card No 361451), indicating that the ZnO

nanoflower/nanorod structures are single phase and crystallize

in the typical wurtzite structure [30]. It was observed that the

intensity of the ZnO(002) diffraction peak is higher compared

to that of the standard ZnO diffraction pattern, which indicates

a c-axis preferred growth direction. In addition, the intensity

of the ZnO diffraction peaks increased following plasma

treatment of the ZnO seed layers because of the increased

density of the ZnO nanoflowers. This is consistent with the

SEM results shown in figures6(c) and (d).

TEM analysis was carried out for further structural char-

acterization. Figures8(a) and (b) provide low-magnification

TEM images of the ZnO nanorods. The typical diameters of

Figure 8. Low-magnification TEM images of the ZnO nanoflower/nanorod structures grown on the (a) as-prepared ZnO seed layers and (b)

plasma-treated ZnO seed layers. HRTEM images of the ZnO nanoflower/nanorod structures grown on the (c) as-prepared ZnO seed layersand (d) plasma-treated ZnO seed layers with their corresponding FFT pattern (inset).


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these ZnO nanorods are in the range of 3090 nm, which is

consistent with those observed using the FE-SEM and shown

in figures6(a) and (b). However, the length (200500 nm) of

the ZnO nanorods was not consistent with FE-SEM results.

This is likely due to the fact that the ZnO nanorods were

mechanically broken when peeled away from the substrates by

sonication. High-resolution TEM (HRTEM) images of a ZnOnanorod are shown in figures8(c) and (d). The lattice spacing

was measured to be 0.26 nm, which matches well the literature

value for the distance between (0001) planes in the ZnO

crystal and indicates that the ZnO nanorods preferentially

grow along thec-axis [0001] direction. Fast Fourier transform

(FFT) patterns (insets of figures8(c) and (d)), performed on

individual nanorods, prove that the ZnO nanorods are single

crystalline.

The optical properties of the ZnO nanoflowers/nanorods

were studied using spontaneous Raman scattering. ZnO has a

wurtzite crystal structure and belongs to the C46v space group

with two formula units per primitive cell, where all the atoms

occupy the C3v symmetry. Near the center of the Brillouinzone, group theory predicts the existence of the following

phonon modes: = A1 + 2B1 + E1 + 2E2. The B1 modes

are forbidden while the A1,E1, and E2 modes are (Raman)

allowed. Additionally, the A1 and E1 are also infrared active

and split into two components, i.e. transverse optical (TO) and

longitudinal optical (LO) components [31]. The frequencies

of Raman active phonon modes in ZnO are as follows:

E2(low) = 101 cm1,E2(high) = 437 cm

1,E1(TO) =407 cm1,E1(LO) = 583 cm

1,A1(TO) =380 cm1, and

A1(LO) = 574 cm1 [32]. Figure 9 shows the Raman

scattering spectra of the bare PEN substrate (figure 9(a))

and the ZnO nanoflowers/nanorods (figures 9(b) and (c)).A sharp and strong peak at 438 cm1 is assigned to the

E2 (high frequency) optical phonon mode of the ZnO,

which is characteristic of the wurtzite hexagonal phase of

ZnO [33]. Two weak peaks located at 333 cm1 and 382 cm1

are assigned to the E2 (high)E2 (low) (second-order

multiple-phonon scattering) andA1(TO) modes, respectively.

The multiple-phonon scattering is likely due to the quantum

confinement effects in the ZnO nanostructures [34]. The

absence of the E1 (LO) mode at 583 cm1 indicates that

the ZnO nanoflowers/nanorods are of good crystal quality,

because theE1(LO) mode is associated with structural defects

(zinc interstitials or oxygen vacancies) and impurities in the

ZnO crystal.

4. Conclusions

Demonstrated in this work is the synthesis of ZnO

nanoflowers/nanorods on PEN substrate using a hydrothermal

method. Spin-deposited ZnO seed layers were treated

with an APPJ array before the growth of the ZnO

nanoflowers/nanorods. The resultant individual nanoflowers

were composed of a number of hexagonal nanorods, which

grew radially in many directions. The plasma treatment

induced a significant increase in the height and density of

the ZnO nanoflowers/nanorods because the plasma effectivelyincreased the surface energy and roughness of the seed layers

Figure 9. Raman scattering spectra of the (a) bare PEN substrateand the ZnO nanoflowers/nanorods grown on the (b) as-preparedZnO seed layers and (c) plasma-treated ZnO seed layers.

while barely affecting the crystal shape and phase of theZnO nanoflowers/nanorods. The XRD and Raman scatteringmeasurements indicated that the ZnO nanoflowers/nanorodshave good crystal quality with a hexagonal wurtzite structure.

The multiple-stack high-density ZnO nanoflowers/nanorodsare easily obtained through a simple plasma treatmentof the seed layers during conventional hydrothermalsynthetic procedures, making APPJ treatment a very usefulenhancement for flexible electronics on plastic substrates.

Acknowledgments

The authors wish to thank the Samsung Advanced Instituteof Technology at Samsung Electronics for financial support.The authors also thank Dr H Qian of the Clemson UniversityElectron Imaging Facility for technical assistance.

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Intense plasma emission induced by jet-to-jet coupling in atmosphericpressure plasma arrays

Sung-O Kim,1,a) Jae Young Kim,1 Do Yeob Kim,1 and John Ballato21Holcombe Department of Electrical and Computer Engineering and the Center of Optical Materials Scienceand Engineering Technologies (COMSET), Clemson University, South Carolina 29634, USA2Department of Materials Science and Engineering and The Center of Optical Materials Scienceand Engineering Technologies (COMSET), Clemson University, South Carolina 29634, USA

(Received 14 August 2012; accepted 9 October 2012; published online 24 October 2012)

Intense plasma emissions were achieved via jet-to-jet coupling in a multi-tube array-based plasma

device in ambient air. The plasma array device consisted of a central glass tube encircled by an

array of hollow glass tubes. A single plasma jet was induced via jet-to-jet coupling and enabled

significantly increased plasma emission despite a negligible change in power consumption. An

increase in the number of outer tubes yielded a greater number of charged particles involved in the

plasma process and resulting in the achievement of higher plasma emission in the coupled system.

VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4764022]

While cold atmospheric pressure plasmas have the great

advantages of simple structure, easy fabrication, low temper-

ature emissions, and high chemical reactivity,15 their low

energetic properties, which are due to the weakly ionizeddischarge state, limits their applicability. Recent research

suggests this deficiency is overcome through the use of

two or more plasma jets, in which a jet-to-jet coupling

occurs between adjacent atmospheric pressure plasma jets

(APPJs).610 While operating in the honeycomb-shaped

plasma array configuration,810 the plasma jet generated in

the central tube formed its own plasma plume in the ambient

air, and the neighboring plasma jets generated in the outer

tubes provided charged particles to the central plasma plume.

Thus, the central plasma plume was reinforced by an abun-

dance of charged particles from the outer plasma jets. As a

result, the optical intensity and the electron energy of the

plasma were increased by electrical coupling of charged par-

ticles at atmospheric pressure.8,9

In order to further increase the emission of plasma array

devices, which is a consideration of practical consequence, it is

important to induce stronger jet-to-jet coupling among adjacent

plasma jets. To do so, the number of outer tubes needs to be

increased so that a greater number of charged particles are

introduced into the central plasma plume. When the outer tubes

have smaller diameters the plasma jet array can contain a

greater number of outer tubes in the honeycomb configuration.

In this experiment, two plasma jet array devices are fab-

ricated consisting of a central tube surrounded by an array of

tubes of relatively larger or smaller diameters in order toevaluate the influence of peripheral tube number on jet-to-jet

coupling. Figs.1(a)and1(b)depict the two different types of

plasma jet array devices employed for producing intense

atmospheric pressure plasmas. The plasma jet arrays con-

sisted of one central tube with several other tubes arrayed

around it. The central tube has a larger inner diameter than

the outer tubes. Device I, shown in Fig. 1(a), has a center

quartz tube with a 2 mm inner diameter (ID) and 3 mm outer

diameter (OD) as well as seven outer quartz tubes with 1 mm

ID and 2 mm OD. To increase the number of outer tubes,

hollow-core optical fibers were used as the outer tubes in a

second plasma jet array, Device II, shown in Fig.1(b). In De-

vice II, the center quartz tube has a 1 mm ID and 2 mm OD.The peripheral hollow optical fibers have a 200 lm ID and

700lm OD. Fourteen hollow optical fibers functioned as the

outer tubes for Device II. In both devices, copper tape was

used as the powered electrode and was wrapped around the

FIG. 1. Two atmospheric pressure plasma jet array devices employed in this

work: (a) Device I: central quartz tube of 2 mm ID and 3 mm OD surrounded

by seven outer tubes of 1 mm ID and 2 mm OD. (b) Device II: central quartz

tube of 1 mm ID and 2 mm OD surrounded by fourteen hollow optical fibers

of 200lm ID and 700lm OD.a)

Electronic mail: [email protected]. Fax: 1-864-656-5910.

0003-6951/2012/101(17)/173503/5/$30.00 VC 2012 American Institute of Physics101, 173503-1

APPLIED PHYSICS LETTERS101, 173503 (2012)


12/16

entire array 10 mm from the end of the tubes. Indium tin ox-

ide (ITO) coated glass with a thickness of 0.8 mm was placed

10 mm from the ends of the devices with the glass side fac-

ing the plasma jets to serve as a ground electrode. The

atmospheric plasma system employed in this work is similar

to that described previously.48 The gas flow was split in two

so that the gas flow rates in the central tube and the outer

tubes could be independently controlled. High purity helium

(99.997%) was used as a discharge gas. The sinusoidal volt-age applied to the devices had a peak value of 7.5 kV with

various frequencies (27.5 kHz to 40 kHz with intervals of

2.5 kHz). The jet-to-jet coupling effect between adjacent

APPJs was optimized by the adjustment of gas flow rates

through the center and outer tubes.

In order to confirm plasma jet-to-jet coupling behavior

between adjacent APPJs, the emission properties of the

plasma arrays were investigated with and without the gas

flowing through the outer tubes while all other driving condi-

tions were held constant. Figure 2 shows the plasma emis-

sion and current flow through the two plasma devices with

and without the gas flowing through the outer tubes with the

same central gas flow rate of 1.5 standard liter per minute

(slm) and the same driving conditions (sinusoidal voltage of

7.5 kV with frequency of 31 kHz). Since the discharge can be

ignited with helium flow but not in air at 7.5 kV, plasma

plumes did not occur from the outer tubes when gas was not

flowing through them despite having voltage applied to the

electrode that was in contact with the outer tubes. This is

shown in Figs. 2(a)and2(c). Thus, the resultant plasma jet

from the central gas flow is identical to a single plasma jet.

However, when appropriate gas flows are applied to the outer

tubes of the arrays, the optical intensity of the central plasma

jet is increased due to significant coupling from outer plasma

jets. This is observed in both plasma devices as shown in

Figs.2(b)and 2(d). When the gas flow rates are 700 and 140

standard cubic centimeters per minute (sccm) for Devices Iand II respectively, the maximum optical intensities are

achieved. The displacement currents, shown as sinusoidal

waveforms, are almost identical regardless of the operation

of the outer plasma jets. Conversely, the discharge currents,

shown as current peaks, increase when the outer plasma jets

couple. Since the discharge current is much smaller than the

displacement current, the overall current flowing is approxi-

mately the same in terms of the total consumed power.

Under these experimental conditions, the consumed power

of both increased by only 2 W (from 33 W to 35 W in Device

I and from 37 W to 39 W in Device II) with the addition of

the outer APPJs. As the small amount of discharge current

indicates, most of the power consumption was for charging

and discharging the capacitive device and only around 2.5 W

was used for generating the plasma jets. This suggests that

even if the plasma emission significantly increases, the

power consumption of the plasma device would not change

much for the same gas flow and electrical driving conditions.

FIG. 2. Electrical characteristics and optical intensity of plasma emission from the single and intense plasma jet modes in Devices I and II: (a) single plasma

jet and (b) intense plasma jet in Device I and (c) single plasma jet and (d) intense plasma jet in Device II.

173503-2 Kim et al. Appl. Phys. Lett.101, 173503 (2012)


13/16

In general, when the frequency of the driving voltage

increases more space charges temporarily remain as priming

particles in the discharge volume and affect the next discharge

cycle by either increasing plasma emission or decreasing driv-

ing voltage.1113 In this work, we investigate the dependency

of the plasma jet-to-jet coupling effect between adjacent

APPJs on driving frequency. The driving frequency was

increased with a constant driving voltage of 7.5 kV, and the

discharge characteristics of the plasma array devices, such asplasma emission and discharge delay, were observed.

Figure S1 in supplementary material16 shows the optical

intensity of the plasma emission from Device I with

increases in driving frequency from 27.5 kHz to 40 kHz in

intervals of 2.5 kHz. The optical intensity in the positive half

cycle of the voltage waveform is shown to be higher than

that in the negative half cycle. This is the case for both

plasma jets, with and without outer gas flow. This difference

in optical intensities is caused by the configuration between

the powered and ground electrodes. These plasma systems,

which consist of a plasma array with a single electrode and

an outside ground electrode, can be classified as point-to-

plane discharge configurations. The difference in optical

intensities between rising and falling slopes of the voltage

waveform is a typical discharge phenomenon of point-to-

plane barrier discharges driven by ac voltages: the streamer-

like discharge mode in the positive half-period and the

diffuse-like discharge mode in the negative half-period.14,15

Consequently, when the powered electrode plays the role of

an anode and the ITO ground electrode plays the role of a

cathode, stronger plasmas are generated than when the elec-

trode roles are reversed.

To investigate the discharge properties related to the

applied voltage waveform in greater detail, the optical inten-

sities of the plasma emission from Device I were separatedinto two groups as shown in Fig. 3. One group is the optical

intensity of the plasma emission during the positive half

cycles of the voltage waveform (Fig. 3(a)), and the other is

optical intensity during the negative half cycles (Fig. 3(b)).

The single plasma jet caption in the figure indicates that

the plasma array only has a single central plasma plume due

to a central gas flow rate of 1.5 slm and no gas flow in the

outer tubes. The intense plasma jet caption in the figure

indicates that the plasma array has an intense plasma jet

caused by plasma jet-to-jet coupling with central gas flow

rate of 1.5 slm and outer gas flow rate of 700 sccm.

Fig.3 shows that the intense plasma jet has a higher op-

tical intensity than the single plasma jet in all experimentalcases, both positive and negative half cycles. As the driving

frequency increased, the optical intensity of plasma emission

also increased in both the single and intense plasma jets.

Interestingly, as the driving frequency increases so does the

margin between the magnitudes of the optical intensities for

the intense plasma jet and the single plasma jet. Increased

driving frequency also creates more volume space charges,

thereby enhancing the plasma coupling behavior between ad-

jacent plasmas.

The optical intensities of the negative half cycles were

markedly increased for higher frequency driving conditions.

When the driving frequency was increased from 27.5 kHz to

40 kHz, the optical intensity of the single plasma jet doubled

during positive half cycles and increased 11 times during neg-

ative half cycles. The optical intensity of the intense plasma

jet tripled during positive half cycles and increased 15 times

during negative half cycles as shown in Fig.3. In other words,

when both plasma jet modes were operated at a frequency of

27.5 kHz the optical intensities during negative half cycles

were negligible; however, when the plasma jets were operated

at a frequency of 40 kHz their optical intensities during nega-

tive half cycles were high enough to compete with the inten-

sities of the positive half cycles. This means that when theplasma jets were operated at a higher frequency, the plasma

emission also showed a streamer-like discharge during the

negative half cycle with assistance from the priming particles.

Figs.4and S2 (supplementary material16) show the opti-

cal intensity of plasma emissions from Device II as the driv-

ing frequency is increased from 27.5 kHz to 40 kHz in

intervals of 2.5 kHz. As with Device I, two plasma jet modes

were compared; a single jet with no outer gas flow and an

intense plasma jet with an outer gas flow rate of 140 sccm.

The resulting discharge characteristics were largely the same

as those from Device I. It is interesting to note that the opti-

cal intensities from Device II were found to be higher than

those from Device I as shown in Figs. 3 and 4. Even though

FIG. 3. Change in optical intensity of the plasma emission from Device I

with an increase of driving frequency from 27.5 kHz to 40 kHz in intervalsof 2.5 kHz. Optical intensity of Device I (a) in positive half cycle of the volt-

age waveform and (b) in negative half cycle.



14/16

Device II is smaller than Device I, the greater number of

outer tubes yielded more charged particles to be delivered to

the central plasma plume. As a result higher plasma emis-

sions can be achieved.

Figs. 5(a) and 5(b) show the normalized discharge

delays with respect to the driving frequency in Devices I and

II, respectively. The discharge delay time is defined as the

time-period from when the continuous sinusoidal voltage

waveform is equal to zero to the time when the optical emis-

sion signal reaches its peak. This discharge delay time is nor-malized to 1/4 of the voltage period to avoid the influence of

varying times for reaching breakdown voltage at different

frequencies as shown in Tables SI and SII in supplementary

material.16 Fig.5 shows that the normalized discharge delay

in the single plasma jet is smaller than that in the intense

plasma jet in all experimental regions; however, this does

not indicate more volume space charges in the single plasma

jet. The normalized discharge delays associated with the

coupling effect indicate that the jet-to-jet coupling effect

requires time for the plasma jets to merge and form the

intense plasma jet.

For the single plasma jet, the normalized discharge

delay is reduced with an increase of the driving frequency.

Since there are more volume space-charged particles at

higher driving frequencies, the corresponding discharge is

generated at a quicker pace. For the intense plasma jet, as

the driving frequency increases from 27.5 kHz to 40 kHz,

the normalized discharge delay of Device I decreased by

40% (from 0.66 to 0.39), whereas the normalized dis-

charge delay of Device II is saturated. This implies that

the device having a greater number of tubes requires more

time to couple and form an intense plasma jet. This reiter-

ates that the plasma devices employed here are efficient

for generating the jet-to-jet coupling effect for more

intense plasmas.In summary, two atmospheric pressure plasma array

devices that produce intense plasma emissions were pro-

posed and their optical and electrical characteristics investi-

gated. The experimental result showed that if the number of

peripheral tubes in the plasma array increases, a greater num-

ber of charged particles participate in the plasma generating

process, resulting in stronger plasma emissions. When the

driving frequency was increased, the intense plasma jet

exhibited a greater increase in plasma emission than the sin-

gle plasma jet. The plasma jet-to-jet coupling induced by the

interaction of charged particles between adjacent APPJs was

found to be a great method for intensifying the plasma emis-

sion compared to single APPJs.

FIG. 4. Change in optical intensity of the plasma emission from Device II

with an increase of driving frequency from 27.5 kHz to 40 kHz in intervalsof 2.5 kHz. Optical intensity of Device II (a) in positive half cycle of the

voltage waveform and (b) in negative half cycle.

FIG. 5. Change in normalized discharge delays of the plasma emission in

single and intense plasma jets as frequency increases in (a) Device I and (b)

Device II.



15/16

1K. H. Becker, K. H. Schoenbach, and J. G. Eden,J. Phys. D 39, R55 (2006).2M. Laroussia and X. Lu,Appl. Phys. Lett. 87, 113902 (2005).3B. L. Sands, B. N. Ganguly, and K. Tachibana, Appl. Phys. Lett. 92,

151503 (2008).4J. Y. Kim, Y. Wei, J. Li, P. Foy, T. Hawkins, J. Ballato, and S.-O. Kim,

Small7, 2291 (2011).5

J. Y. Kim, J. Ballato, P. Foy, T. Hawkins, Y. Wei, J. Li, and S.-O. Kim,

Biosens. Bioelectron. 28, 333 (2011).6

K. Yambe, H. Sakakita, H. Koguchi, S. Kiyama, N. Ikeda, and Y. Hirano,

J. Plasma Fusion Res. Series 8, 13221325 (2009), available at http://

www.jspf.or.jp/JPFRS/PDF/Vol8/jpfrs2009_08-1322.pdf .7Y. Ito, K. Urabe, N. Takano, and K. Tachibana, Appl. Phys. Express 1,

067009 (2008).8

J. Y. Kim, J. Ballato, and S.-O. Kim,Plasma Processes Polym. 9, 253 (2012).

9J. Y. Kim and S.-O. Kim,IEEE Trans. Plasma Sci. 39, 2278 (2011).10J. Furmanski, J. Y. Kim, and S.-O. Kim, IEEE Trans. Plasma Sci. 39, 2338

(2011).11T.-S. Cho, G.-H. Chung, and J.-W. Jung, Appl. Phys. Lett. 92, 221506

(2008).12T.-S. Cho, J.-J. Ko, D.-I. Kim, C.-W. Lee, G. Cho, and E.-H. Choi, Jpn. J.

Appl. Phys. 39, 4176 (2000).13U. Kogelschatz,Plasma Chem. Plasma Process.23, 1 (2003).14

Y. S. Akishev, A. V. Demyanov, V. B. Karalnik, A. E. Monich, and N. I.

Trushkin, Plasma Phys. Rep. 29, 82 (2003).15

M. Petit, A. Goldman, and M. Goldman, J. Phys. D: Appl. Phys. 35, 2969(2002).

16See supplementary material at http://dx.doi.org/10.1063/1.4764022 for

Figures S1 and S2 and Tables SI and SII.



16/16

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