Influences of guide-tube and bluff-body on advanced atmospheric pressure plasmasource for single-crystalline polymer nanoparticle synthesis at low temperatureDong Ha Kim, Choon-Sang Park, Won Hyun Kim, Bhum Jae Shin, Jung Goo Hong, Tae Seon Park, Jeong HyunSeo, and Heung-Sik Tae

Citation: Phys. Plasmas 24, 023506 (2017); doi: 10.1063/1.4975313View online: http://dx.doi.org/10.1063/1.4975313View Table of Contents: http://aip.scitation.org/toc/php/24/2Published by the American Institute of Physics

Influences of guide-tube and bluff-body on advanced atmospheric pressureplasma source for single-crystalline polymer nanoparticle synthesis at lowtemperature

Dong Ha Kim,1,a) Choon-Sang Park,1,a) Won Hyun Kim,2 Bhum Jae Shin,3 Jung Goo Hong,2

Tae Seon Park,2 Jeong Hyun Seo,4 and Heung-Sik Tae1,b)

1School of Electronics Engineering, College of IT Engineering, Kyungpook National University,Daegu 702-701, South Korea2School of Mechanical Engineering, College of Engineering, Kyungpook National University,Daegu 702-701, South Korea3Department of Electronics Engineering, Sejong University, Seoul 143-747, South Korea4Department of Electronics Engineering, Incheon National University, Incheon 406-772, South Korea

(Received 26 October 2016; accepted 19 January 2017; published online 7 February 2017)

The use of a guide-tube and bluff-body with an advanced atmospheric pressure plasma source is

investigated for the low-temperature synthesis of single-crystalline high-density plasma polymer-

ized pyrrole (pPPy) nano-materials on glass and flexible substrates. Three process parameters,

including the position of the bluff-body, Ar gas flow rate, and remoteness of the substrate from the

intense and broadened plasma, are varied and examined in detail. Plus, for an in-depth understand-

ing of the flow structure development with the guide-tube and bluff-body, various numerical simu-

lations are also conducted using the same geometric conditions as the experiments. As a result,

depending on both the position of the bluff-body and the Ar gas flow rate, an intense and broadened

plasma as a glow-like discharge was produced in a large area. The production of the glow-like dis-

charge played a significant role in increasing the plasma energy required for full cracking of the

monomers in the nucleation region. Furthermore, a remote growth condition was another critical

process parameter for minimizing the etching and thermal damage during the plasma polymeriza-

tion, resulting in single- and poly-crystalline pPPy nanoparticles at a low temperature with the

proposed atmospheric pressure plasma jet device. Published by AIP Publishing.[http://dx.doi.org/10.1063/1.4975313]

I. INTRODUCTION

Due to various advantages, such as low cost1–6 and low-

temperature dry process features,7–15 atmospheric pressure

plasma jets (APPJs) are a promising growth source for the

deposition of plasma polymer films16–20 with functional prop-

erties that are well-suited for a wide range of substrates and

applications.14,21,22 Plasma polymer films grown using plasma

sources such as low-pressure and radio-frequency (RF) plas-

mas are known to be pinhole-free, highly cross-linked, insolu-

ble, branched, and adherent to various substrates.23–31

Nonetheless, to obtain high-quality polymer thin films using

plasma sources, especially APPJs, the following difficulties

need to be overcome. First, conventional APPJs are unable to

feed enough energy for nucleation and activation of the mono-

mers due to the weak plasma energy in atmospheric air.32–35

Second, depending on the size of the APPJ device, especially

single jets, the plasma jet diameter is typically only a few

millimeters, meaning that the processing area is limited and

unsuitable for large-area polymerization.35,36 Third, the use of

argon (Ar) gas is more beneficial for large-scale applications

of plasma jet arrays.37–39 However, Ar plasma jet arrays are

difficult to control due to their high breakdown voltage, easy

glow-to-arc transition, and heavier atom mass.40,41 Finally,

understanding the interaction between the plasma and the sub-

strate is critical for suppressing plasma damage during plasma

polymerization.42–44

Thus, to ensure that the monomers are completely frag-

mented in the discharge region, it is important to increase the

plasma energy by suppressing the inflow of ambient air into

the process region; that is, an Ar-rich condition is required to

enhance the plasma energy in the process region for efficient

fragmentation of the monomers. Furthermore, the position of

the substrate for growing polymer thin films needs to be con-

sidered in relation to the intense plasma region; that is, a

remote substrate location is required to avoid severe plasma

damage induced by energetic charged particles.

The current authors recently reported a new polymer

synthesis method using a novel APPJ device, where poly-

crystalline plasma-polymerized aniline (pPANI) nano-

materials were successfully grown under Ar dominant gas

conditions by simply shielding the plasma generation region

from the ambient air.45 However, to understand the detailed

growth mechanism of high-quality plasma polymers when

using high-pressure plasma jets, the plasma polymerization

process and numerical procedure of gas fluid physics need to

be investigated when varying several critical parameters,

such as the position of the polytetrafluoroethylene (PTFE)

a)D. H. Kim and C.-S. Park contributed equally to this work.b)Electronic mail: hstae@ee.knu.ac.kr.

1070-664X/2017/24(2)/023506/15/$30.00 Published by AIP Publishing.24, 023506-1

PHYSICS OF PLASMAS 24, 023506 (2017)

bluff-body (hereinafter, bluff-body) in the glass tube, gas

flow rate (or gas velocity), and electrical interaction of the

plasma plume with the surface of the substrate.

Accordingly, this study experimentally and numeri-

cally examined the effects of the guide-tube and bluff-

body on an advanced APPJ device for the low-temperature

synthesis of high-density plasma-polymerized pyrrole

(pPPy) with a single-crystalline characteristic. Thus, we

examined the effects of three process parameters on gener-

ating a uniform and strong glow-like discharge in the pro-

posed APPJ device for large-area treatment and improving

the crystalline characteristic of pPPy. The three process

parameters were the position of the bluff-body with respect

to the guide-tube, the Ar gas flow rate, and the remoteness

of the substrate from the intense and broadened plasma.

The gas fluid and plasma characteristics were measured

using numerical procedures, optical emission spectroscopy

(OES), infrared spectroscopy (IR), and the currents.

Fourier transform infrared spectroscopy (FT-IR), field

emission-scanning electron microscopy (FE-SEM), and

transmission electron microscopy (TEM) were also used to

analyze the pPPy nano-materials.

The experimental results showed the production of

unique plasma characteristics in the nucleation region of the

advanced APPJs, that is, the streamer-like discharge or

glow-like discharge varied considerably depending on both

the position of the bluff-body with respect to the guide-tube

and the Ar gas flow rate. Moreover, the crystalline quality of

the pPPy grown using the advanced APPJs depended

strongly on the remoteness of the substrate from the uniform

and intensified glow-like discharge.

II. EXPERIMENTAL

Figure 1 shows a schematic diagram of the experimental

setup of the advanced APPJs used in this study. The three jet

tubes, arranged in a triangle shape, were wrapped with cop-

per tape as a powered electrode, 10 mm from the end of the

jets, which produced a more compact design as each jet was

in physical contact with the adjacent jets. Each jet tube was

13 cm in length, with an inner diameter of 1.5 mm and an

outer diameter of 3 mm. The center-to-center distance

between two adjacent tubes was exactly 3 mm. As shown in

Figure 1, one of the critical components of the proposed

APPJs is the glass guide-tube (hereinafter, guide-tube), con-

stituting the nucleation region where the pyrrole monomers

are cracked by the Ar plasma. Here, the guide-tube was

60 mm in length, with an inner diameter of 20 mm. Another

critical component is the bluff-body, as the position of the

bluff-body with respect to the guide-tube has a significant

influence on whether intense and broadened plasma can be

produced in the nucleation region. Here, the bluff-body had

an outer diameter of 15 mm. The substrates employed for the

plasma polymerization included glass, Si wafers, and poly-

ethylene terephthalate (PET). The substrates were mounted

on the bluff-body. High purity Ar gas (99.999%) was used as

the discharge gas for the plasma generation, and its flow rate

was varied from 700 to 1600 standard cubic centimeters per

minute (sccm) at an interval of 300 sccm. The liquid pyrrole

monomer (Sigma-Aldrich Co., St. Louis, Missouri, USA,

Mw¼ 67 g mol�1) was vaporized using a glass bubbler that

supplied the Ar gas at a flow rate of 130 sccm. A sinusoidal

power source was connected to the powered electrode via a

driving circuit with a peak value of 12 kV and a frequency of

FIG. 1. Schematic diagram of the

experimental set-up employed in this

study.

023506-2 Kim et al. Phys. Plasmas 24, 023506 (2017)

30 kHz. A high-voltage probe (Tektronix P6015A) and cur-

rent probe (Pearson 4100) were connected between the

power source via an inverter circuit and oscilloscope

(LeCroy WaveRunner 64Xi) to measure the applied voltage

and total current, respectively. In the driving circuit, the

inverter amplified the low primary voltage to reach a high

secondary voltage. As a result, the driving circuit generated

a sinusoidal voltage of several tens of kilovolts, with a fre-

quency of several tens of kilohertz. A photo-sensor amplifier

(Hamamatsu C6386-01) was employed to analyze the plasma

infrared (IR) emission in the nucleation region. The

wavelength-unresolved optical emission waveform from

the photo-sensor amplifier with a range of 400–1100 nm (mid-

dle range) was connected to an oscilloscope. A 1-mm-thick

glass sheet was placed in front of the photo-sensor amplifier

to eliminate any external signals. Since the measurement lead

of the optical fiber was attached just below the substrate, the

measured plasma infrared (IR) intensity varied depending on

the remoteness of the substrate from the plasma plume.

All the photographs of the devices and plasma plumes

were taken using a DSLR camera (Nikon D5300) with a

Macro 1:1 lens (Tamron SP AF 90 mm F2.8 Di). An optical

emission spectrometer (OES, Ocean Optics, USB-4000UV-

VIS) was used to identify the diverse reactive species in the

plasma. Instead of measuring the plasma density and electron

temperature, a photo-sensor amplifier and OES techniques

were used to measure and analyze the optical intensities and

spectra of the reactive nitrogen/oxygen peaks, respectively,

to estimate the variations in the plasma energy. Before the

plasma polymerization, the substrates with identical sizes of

10 mm � 10 mm were ultra-sonically cleaned in 99.99% ace-

tone, isopropanol, and distilled (DI) water for 20 min, respec-

tively, to remove any contaminants from the surface of the

substrates. The surface temperature of the substrates was

measured using a special glass tube with an infrared ther-

mometer (Fluke, 568 IR Thermometer).

The study investigated the effects of varying three pro-

cess parameters to enable the advanced array APPJs to

produce broadly distributed intense glow-like plasmas for

large-area treatment. Based on the optimized results for the

proposed APPJs, the optimized growth process conditions

were identified for high-quality plasma polymer thin films,

that is, the conditions were optimized for improving the crys-

talline quality of the plasma polymer thin films. The detailed

process parameters are given in Figure 2(a) and Table I. To

investigate the blocking effect of the bluff-body, the first

process parameter involved varying the position of the bluff-

body with respect to the guide-tube at a constant Ar gas flow

rate of 1300 sccm; that is, in case I, the bluff-body was

located outside the guide-tube, in case II, the bluff-body was

located exactly at the end of the guide-tube, and in cases

III–V, the bluff body was located inside the guide-tube. The

second process parameter involved varying the Ar gas flow

rate from 700 to 1600 sccm at an interval of 300 sccm for

case III, where the bluff-body was located about 5 mm inside

the guide-tube: that is, all the other experimental conditions

were exactly the same as those for case III, except for the Ar

gas flow rate.

Figure 2(b) shows a cross-sectional schematic diagram

of the three jets and guide-tube comprising the proposed

APPJs for large-area treatment. In the case of conventional

APPJs without a guide-tube or bluff-body, the plasma is only

produced within the area of the three array jets (or bundle of

3 glass tubes) due to the directional characteristic of the

streamer-like discharge.46–49 However, in our previous study,

the streamer-like plasma was already proven to be unsuitable

for growing high-quality plasma polymer thin films.45 In con-

trast, with the proposed APPJs, the plasma produced in the

nucleation region can be transited from a narrow streamer-

like discharge into a broadened glow-like discharge by

properly adjusting the process parameters. Importantly, when

producing the broader glow-like plasma in the nucleation

region, the plasma area is dramatically enlarged (about

60-fold increase) when compared with that with the narrow

streamer-like plasma produced by conventional APPJs with

three arrays. Plus, the experiments showed that the proper

combination of the guide-tube with the bottom played a sig-

nificant role in producing largely distributed glow-like

plasma in the nucleation region, thereby also improving the

crystalline quality of the plasma polymer thin films.

Figure 3 shows the changes in the plasma images pro-

duced in the nucleation region when varying the process

parameters. As shown by case I in Figure 3(a), the short

plasma plumes produced in the limited region looked like the

streamer discharge typically observed with conventional

array jets. In case II, the plasma plume still appeared to be a

streamer discharge, even though its intensity was slightly

enhanced and its length slightly extended compared to that of

case I. However, in case III, where the bluff-body was located

inside the guide-tube, strong plasma plumes were produced

and extended much farther downstream, confirming that the

conditions of case III caused a dramatic transition of the

plasma produced from a narrow streamer-like discharge into

a broadened and intense glow-like discharge. Thus, the

proper combination of the bluff-body with the guide-tube

provided Ar-rich conditions that increased the plasma energy

in the nucleation region by suppressing the inflow of ambient

air into the plasma production region. As a result of minimiz-

ing the quenching effect, an intensified glow-like plasma

cloud was produced, as shown by case III in Figure 3(a).

Figure 3(b) shows the plasma images produced in the

nucleation region of the proposed APPJs according to the Ar

gas flow rate under the process parameter conditions of case

III. In the case of a low gas flow rate below 1300 sccm (i.e.,

case III A: 700 sccm and case III B: 1000 sccm), no intense

or broadened plasma was produced even though the bluff-

body was located inside the guide-tube. Meanwhile, when

the gas flow rate was over 1300 sccm (i.e., case III C: 1300

sccm and case III D:1600 sccm), the intense and broadened

plasma was produced in case III when the bluff-body was

located inside the guide-tube. This result shows that the Ar

gas flow rate was also a significant process parameter for

producing intense glow-like plasma. In other words, increas-

ing the Ar gas flow rate under the conditions of case III

increased the Ar species and monomers in the nucleation

region, resulting in an intense glow-like discharge that is

needed for fully cracking the monomers in the nucleation

023506-3 Kim et al. Phys. Plasmas 24, 023506 (2017)

region. Moreover, to avoid thermal and etching damage, the

substrate needs to be located away from the intense plasma

region during the plasma polymerization.

Thus, cases IV and V in Figure 3(c) and Table I were

used to check the effect of another process parameter as

regards minimizing the plasma damage during plasma poly-

merization. In cases IV and V, as the substrate approached

the intense plasma region, more intense glow-like plasmas

were produced, yet additional streamer-like discharges were

also observed in the immediate vicinity of the substrate, as

shown in Figure 3(c). Direct contact between the substrate

and the plasma plume can cause thermal damage or etching,

eventually aggravating the quality of the polymer films

grown in the APPJs.

TABLE I. Parametric studies employed in this study.

Variables Parametric cases

Position of bluff body (distance from end

of guide-tube) (Ar gas flow rate: 1300 sccm

(Case I) Outside 5 mm

(Case II) 0 mm

(Case III)a Inside 5 mm

(Case IV) Inside 25 mm

(Case V) Inside 40 mm

Gas flow rate (under case III) (Case III A) Ar 700 sccm

(Case III B) Ar 1000 sccm

(Case III C)a Ar 1300 sccm

(Case III D) Ar 1600 sccm

aNote that case III and case III C are exactly the same experimental condition.

FIG. 2. (a) Various cases employed for

parametric studies relative to various

positions of bluff body with respect to

guide-tube and Ar gas flow rate, where

case I is outside 5 mm (without bluff

body), case II is 0 mm, case III is

inside 5 mm, case IV is inside 25 mm,

and case V is inside 40 mm and under

case III relative to various gas flow

rates (case III A: Ar 700 sccm, case III

B: Ar 1000 sccm, case III C: Ar 1300

sccm, and case III D: Ar 1600 sccm)

and (b) comparison of the cross-

sectional area of the plasma produced

by conventional jets with only three

array jets (case I) and proposed APPJs

(case III), where A denotes the plasma

area in case I and B denotes the plasma

area in case III.

023506-4 Kim et al. Phys. Plasmas 24, 023506 (2017)

Consequently, unlike cases IV and V (non-remote con-

dition or coupling condition), case III (¼case III C) repre-

sents a remote mode that is suitable for the efficient

deposition of plasma polymerized nanoparticles without

etching or thermal damage.

III. RESULTS AND DISCUSSION

A. Numerical procedure and results

To gain an in-depth understanding of the flow structure

development within the guide-tube and bluff-body, various

numerical simulations were also conducted using the same

geometric conditions as the experiments. Herein, while the

numerical simulations only depict the non-plasma flow field,

the plasma added flow behavior can be inferred on the basis

of available computational results.

Figure 4 shows the entire computational domain with

the geometric conditions, including three main parts: the

inflow tube, mixing chamber, and bluff-body, corresponding

to the bundle of three glass tubes, guide-tube, and bluff-body

in the experiments, respectively. To determine the effects of

the inserted bluff-body on the flow structure, five bluff-body

positions from the outlet were selected, similar to the experi-

mental conditions of cases I–V. The incoming fluid into the

FIG. 3. Changes in plasma images produced in nucleation when varying process parameters for proposed APPJs: (a) positions of bluff body with respect to

guide-tube: case 1; outside 5 mm (without bluff body), case II; 0 mm, and case III; inside 5 mm, (b) variation of Ar gas flow rate: case III A; 700 sccm, case III

B; 1000 sccm, case III C; 1300 sccm, and case III D; 1600 sccm, where all the other conditions are exactly the same as case III in (a), and (c) in cases IV and

V, bluff body is inserted further into guide-tube: case IV; inside 25 mm and case V; inside 40 mm.

023506-5 Kim et al. Phys. Plasmas 24, 023506 (2017)

mixing chamber (guide-tube) was prescribed as a perfectly

stirred mixture of Ar (1300 sccm) and the Ar þ monomer

(130 sccm) with 300 K ejecting from the inflow tube (bundle

of three jets). The details of the geometric and boundary con-

ditions are summarized in Table II. As plotted in Figure 4,

the current geometric feature was very similar to a small-

scale dump or can-type combustor. According to previous

studies of such meso- or micro-scale combustors, a turbu-

lence model is recommended to obtain more accurate numer-

ical results than assuming a laminar flow regime when the

Reynolds number is more than 400.50–52 In the present study,

the Reynolds number based on the inflow tube diameter (Dt)

was about 700. Moreover, not referred in this paper, an

unsteady simulation was performed and the results were

compared with the steady simulation. As a result, since the

unsteadiness of the flow feature was determined to be very

weak, the numerical simulations were only conducted for

steady and incompressible turbulent flows, and the governing

equation for the continuity and momentum is as follows:

@

@xiqUið Þ ¼ 0; (1)

@

@xjqUiUjð Þ ¼ �

@P

@xiþ @sij

@xj� @

@xjqu0iu

0j

� �; (2)

where q, Ui, P, sij, and u0iu0j denote the density, velocity com-

ponents, pressure, stress tensor, and Reynolds stress, respec-

tively. When using Ansys FLUENT 13.0,53 the SIMPLEC

Algorithm was applied for the pressure-velocity coupling

and the second-order upwind scheme used for all the equa-

tions. In Equation (2), the turbulent fluctuation driven

Reynolds stress term (uiuj) required additional modelling to

solve the turbulent flows. As various turbulent models are

available, selecting the proper turbulence model is crucial to

achieve an exact prediction of the turbulent flows. Moreover,

due to a sudden expansion of the volume, recirculating flows

occur in the dump region. Such characteristic flow features

are mainly induced by the high strain rate and pressure gradi-

ent intensifying with the help of turbulence anisotropy. Thus,

to obtain an exact solution, the anisotropy of Reynolds stress

must be considered. Therefore, this study applied the

Reynolds stress model (RSM) among the various turbulence

approaches and the transport equation for Reynolds stress is

expressed as follows:53

@

@xkquku0iu

0j

� �¼ Dij þ Pij þ /ij þ eij; (3)

where Dij, Pij, Gij, /ij, and eij denote the turbulent diffusion,

stress production, pressure strain, and dissipation term,

respectively. The modelling details for each term and the

model constants have been omitted.53

FIG. 4. Computational domain and geometric conditions employed in this study.

TABLE II. Details of geometric and boundary conditions for numerical

simulation.

Geometric conditions

Di (inner jet diameter) 1.5 mm

Do (outer jet diameter) 3 mm

Dt (inflow tube diameter) 20 mm

d (distance between jet centers) 6 mm

D (mixing chamber diameter) 9 mm

rb (bluff-body radius) 7.5 mm

L (mixing chamber length) 60 mm

Lt (inflow tube length) 10 mm

Lb (bluff-body position from outlet) Outside 5 mm (case I)

0 mm (case II)

Inside 5 mm (case III)

Inside 25 mm (case IV)

Inside 40 mm (case V)

Boundary conditions

Inlet Velocity inlet, 300 K

(Arþ monomer mixtures, 5.864 m/s)

Outlet Pressure outlet, 300 K

Bluff-body Solid wall, 300 K

Inflow tube wall Glass, 300 K

Mixing chamber wall Glass, 300 K

023506-6 Kim et al. Phys. Plasmas 24, 023506 (2017)

To validate the use of the present numerical method, the

comparable results between the RSM and the experiment for

the jet plumes of case I are plotted in Figure 5. As shown,

the jet plumes of the numerical results are described by the

iso-surface of U/Um¼ 0.4 where Um is the mean streamwise

velocity of a single jet exit. Although the numerical results

of the RSM represent a non-plasma flow condition, the pre-

dicted jet plumes of the RSM agreed well with the experi-

mental results. This feature confirmed that the RSM was

very effective in describing the flow structure in the mixing

chamber.

To check the grid dependency, four grid resolutions

were tested before any further numerical simulations. For

comparison, the distribution of the streamwise velocity on

the center axis with the streamline contour of the x-z plane at

y/Do¼ 0.288 is presented in Figure 6. When the number of

control volumes (CVs) was more than 600 000, the discrep-

ancy depending on the grid resolution for the streamwise

velocity distribution and the formation of recirculating flows

was negligible. Therefore, the grid resolutions in the present

study were maintained at about 650 000–700 000 CVs.

Figure 7 shows the distribution of the streamwise veloc-

ity (U/Uc) at the jet center axis with the streamline contour

of the x-z plane at y/Do¼ 0.288 for different bluff-body posi-

tions (Lb), where Uc is the streamwise velocity of a jet center

at the inflow exit plane. Regardless of Lb, a steep decrease of

U/Uc was observed for 0< x/Do< 3.0 due to a sudden

expansion of the volume. Thereafter, U/Uc gradually

decayed as the flow developed downstream. Because of the

momentum difference between the incoming mixture and the

ambient fluids near the dump region, a wallward motion of

recirculating flows occurred in all cases. Herein, it is worth

noting that the decline pattern of U/Uc and development of

recirculating flows for cases II and III were roughly identical

to case I without the bluff-body. Those cases had recirculat-

ing flows for x/Do¼ 15.0 to a certain distance apart from the

bluff-body. This means there was no mutual effect between

the bluff-body and the flow recirculation zones. Thus, the

flow structures for cases I–III developed alike. Conversely,

the bluff-body had a definite effect on the development of

the flow structure in cases IV and V. Namely, confined and

deformed recirculating flows arose when the bluff-body was

FIG. 5. Comparison of numerical and

experiment results in case I.

023506-7 Kim et al. Phys. Plasmas 24, 023506 (2017)

located further upstream. This was mainly due to insufficient

space for fully developed flows toward downstream. Thus,

the tip of the recirculating flows impinged on the surface of

the bluff-body. From this feature, a sharp decrease of U/Uc

was observed for x¼ 5–7Do (case V) and x¼ 10–11.5Do

(case IV). Simultaneously, the decreased amount of U/Uc

was converted into secondary flow motions due to the con-

servation of momentum, as shown in the following figure.

Figure 8 presents the area-averaged secondary flow

magnitude (Sec) along the streamwise direction and its

contours of the x-z plane at y/Do¼ 0.288 depending on Lb.

Sec is defined as Sec¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiV2 þW2p

=Um, where V and W are

the radial velocity components. Sec for cases IV and V was

about 15%–25% versus Um and its value was sufficiently

large to modify the entire flow structure. For case IV, much

larger secondary flow motions were observed nearby the

bluff-body surface. Due to the increase of vortical motions

induced by such azimuthally developed flows, the impinging

flows on the bluff-body entrained radially and were mixed

with the incoming flow from upstream. This behavior

FIG. 6. Grid dependency for different control volumes.

FIG. 7. Distribution of streamwise velocity at jet center with streamline contour for different bluff-body positions (cases I–V).

023506-8 Kim et al. Phys. Plasmas 24, 023506 (2017)

seemed to be more intensified with the help of strongly

developed secondary flows when the bluff-body was inserted

further inside the mixing chamber, as in case V. Although

secondary flows were also produced near the edge of the

bluff-body in cases II and III, the magnitude was too small to

induce a noticeable change in the flow structure. The result

of the numerical analysis for case III was somewhat dissimi-

lar to that for the experiment, which showed the best perfor-

mance as regards producing intense and broadened plasma.

There were a variety of causes to induce such a discrepancy,

where the major reason seemed to be the non-plasma flow

conditions. The use of a plasma jet for the inflow condition

when conducting further numerical simulations would allow

a more extended flow recirculation zone also affecting the

bluff-body. Accordingly, further vigorous secondary flow

motions and flow mixing to guarantee an intense plasma

regime could be ascertained in a similar way to describe the

phenomenon for cases IV and V. Therefore, the current

numerical results were very meaningful to provide informa-

tion on the flow structure for predicting plasma-added flow

behaviors.

Figure 9 shows the static pressure contours of the x-z

plane at y/Do¼ 0.288 and the distribution of the area-

averaged static pressure magnitude in the streamwise direc-

tion for Lb. As shown, the static pressure on the bluff-body

increased when shifting the position of bluff-body upstream.

FIG. 8. Comparison of (a) secondary flow contour and (b) area-averaged secondary flow magnitude for different bluff-body positions (cases I–V).

FIG. 9. Comparison of (a) static pressure contour and (b) area-averaged static pressure magnitude for different bluff-body positions (cases I–V).

023506-9 Kim et al. Phys. Plasmas 24, 023506 (2017)

For cases I–III, the static pressure increased slightly in the

flow-developing zone and near the bluff-body. However,

cases IV and V showed a definite increase in the static pres-

sure in the vicinity of the bluff-body. This means that the

highly convective flows toward downstream were strongly

impeded by the bluff-body before they reached a fully devel-

oped state. Therefore, the impinging flow appeared with a

large amount of static pressure and secondary flow motions.

In particular, the mixture near the bluff-body was attributed

to the increased static pressure. Thereafter, the newly incom-

ing mixture from upstream impinged again on the remaining

previous mixture, resulting in a highly reactive and mixed

flow. This feature explains the streamer-like discharge on the

surface of the bluff-body in cases IV and V rather than the

intense plasma, as shown in Figure 3(c).

B. Optical, electrical, and discharge characteristics

Figures 10(a) and 10(b) show the applied voltages, total

currents, and IR emission intensities measured in the nucle-

ation region of the proposed APPJs according to the various

positions of the bluff-body. In Figure 10(a), the voltages

were relatively high for cases I–III when the bluff-body was

spatially away from the plasma region, whereas the voltages

decreased in cases IV and V when the bluff-body was closer

to the plasma region. In particular, in case V when the bluff-

body was in contact with the intense plasma region, the

applied voltage was the lowest, meaning that the plasma pro-

duced was easily transited into the arc plasma when applying

a voltage greater than the one in case V. As shown in Figures

10(a) and 10(b), when compared with cases I and II, the IR

emission intensity in case III was increased significantly

without any change in the discharge voltage and current. It is

worth noting that the optical emission peak shifted to the

left, when compared with the case without a bluff-body

(case I). However, in cases IV and V, the IR intensities sig-

nificantly increased even with a lower voltage. These abrupt

increases in cases IV and V in Figure 10(b) imply that ener-

getic charged particles flowed into the substrate from the

plasma region, inducing severe plasma damage during the

plasma polymerization.

Figure 11(a) shows the optical emission spectra (OES)

in the wavelength region ranging from 300 to 880 nm mea-

sured for the five different cases I–V in the nucleation region

of the proposed advanced APPJs. In particular, the detailed

optical emission spectra in Figure 11(a) were magnified for

the wavelengths ranging from 300 to 420 nm for cases I–III

in Figure 11(b) (remote condition), and for cases IV and V in

Figure 11(c) (coupling condition). The OES data in Figure

11 show various peaks, such as excited N2, Ar, OH, and car-

bonaceous (CN) species, existing in the plasma plumes when

applying the voltages in Figure 10(a). The various N2 and

OH peaks exhibited a higher concentration of reactive nitro-

gen species (RNS) and reactive oxygen species (ROS),

respectively, present in the APPJs with the various positions

of the bluff-body; both of these free radicals have been

shown to play an important role in various biological/medi-

cal and industrial applications2,8,35 and imply the presence of

a more efficient, useful, and energy-dense plasma. As shown

by the full spectrum measurements in Figure 11(a), various

excited nitrogen second positive system (SPS) (N2; 337, 357,

and 380 nm) peaks,54–57 a carbonaceous (CN; 388 nm)

peak,45,58,59 and excited Ar peaks (Ar: 695–850 nm)60 were

significantly increased, especially in case III when the bluff-

body was appropriately located inside the guide-tube. The

significantly increased Ar peaks (Ar: 695–850 nm) in case III

illustrate the excited Ar-rich condition in the nucleation

region of the APPJs, which confirms that the monomers

could be fully cracked, satisfying the necessary condition for

growing plasma polymer thin films, as shown in Figure

11(a). In addition, in Figure 11(b), the strong nitrogen SPS

peaks observed in the remote condition of case III were evi-

dence of a plasma energy increase, although no oxygen spe-

cies peaks60–62 were observed. Meanwhile, in Figure 11(c),

oxygen species peaks (OH; 308 nm) were observed under the

coupling condition of cases IV and V, which may have been

FIG. 10. (a) Applied voltages, total currents, and (b) plasma infrared (IR)

emission intensities measured in the nucleation region of APPJs relative to

various positions of bluff body (cases I–V).

023506-10 Kim et al. Phys. Plasmas 24, 023506 (2017)

caused by an increase in the local thermal energy induced by

the streamer-like discharge ignition presumably due to the

location of the substrate closer to the intense plasma plume.

Furthermore, the CN (388 nm B2R! X2R) peak from carbo-

naceous species, which was emitted during the nucleation

processes of the pyrrole monomer, was higher in cases III

and IV (Figures 11(b) and 11(c)) with the bluff-body. This

means that the presence of carbonaceous species in the pro-

posed APPJs was evidence of sufficient nucleation and frag-

mentation of the pyrrole monomer.

C. Functionalization and characterization of newlysynthesized pPPy nanoparticles using APPJs

Figure 12 shows top and cross-sectional SEM images of

the pPPy nanofibers with nanoparticle thin films grown for

30 min on a glass substrate under the five different cases

(I–V) of the proposed APPJs. The surface temperature of the

substrates during the plasma polymerization under the

remote conditions (cases I–III) was about 27, 30, and 30 �C,

respectively.63,64 Whereas, under the coupling conditions,

with direct contact between the substrate and the intense

plasma plume (cases IV and V), the surface temperature of

the substrates was about 65 �C and 75 �C, respectively. The

morphology and cross-sectional images of the pPPy

nanoparticles were characterized by field emission-SEM

(FE-SEM, Hitachi SU8220).

In cases I and II, no nanoparticles or amorphous states

were observed as the plasmas produced during the nucleation

were too weak to crack the monomers efficiently. However,

in case III when the bluff-body was appropriately located

inside the guide-tube, many nanofibers with nanoparticles

were observed to be linked together in uniform and upright

networks, meaning that uniform nanofibers and nanoparticles

were efficiently synthesized under case III for the proposed

APPJs. In general, plasma polymer structures have many

irregular cross-linked networks and porous networks.

However, the SEM image of case III in Figure 12 confirmed

polymer structures with regular networks, not irregular

cross-linked networks. It was noticeable that the height and

density of the pPPy nanofibers/nanoparticles were signifi-

cantly increased by the advanced APPJ technique at a low

temperature. Whereas, in cases IV and V, many nanofibers

and nanoparticles were observed to be damaged and melted

down, which was induced by the severe ion bombardment

and thermal damage due to the strong streamer discharge. In

addition, as shown in Figure 12, the deposition rate in case

III was increased significantly at about 0.93 lm min�1 under

the low-temperature process, meaning that the nano-size

FIG. 11. (a) Optical emission spectra

in the region of wavelength ranging

from 300 to 880 nm measured for five

different cases (I–V) in nucleation

region of proposed APPJs. In (a), opti-

cal emission spectra magnified in the

region of wavelength ranging from 300

to 420 nm are shown in (b) for cases

I–III (remote condition) and (c) for

cases IV and V (coupling condition).

023506-11 Kim et al. Phys. Plasmas 24, 023506 (2017)

polymer grew rapidly during the plasma polymerization in

the proposed APPJs without a thermal solution process.

Figure 13 shows the transmission electron microscopy

(TEM) images of the pPPy nanoparticles prepared under

case III of the proposed APPJs. The high-resolution TEM

(HR-TEM) images and fast Fourier transform selected area

electron diffraction (FFT-SAED) patterns65–70 were taken

with a Titan G2 ChemiSTEM Cs Probe (FEI Company,

Hillsboro, Oregon, USA) transmission electron microscope,

operating at 200 kV. The TEM samples were prepared by

FIG. 12. Top and cross-section views of

scanning electron microscopy (SEM)

images of the plasma-polymerized pyr-

role (pPPy) nanoparticle thin film pre-

pared via APPJs after 30-min deposition

relative to various positions of bluff

body (cases I–V). Scale bar¼ 2lm.

023506-12 Kim et al. Phys. Plasmas 24, 023506 (2017)

depositing a 6 ll solution of pPPy nanofibers with nanopar-

ticles (ultrasonically dispersed in DI water) on carbon-coated

copper grids, and dried in air. As shown in Figures 13(a) and

13(b), pPPy nanoparticles with a diameter range of 5–25 nm

were clearly observed. The FFT-SAED pattern of pPPy

nanoparticles (Figures 13(a) and 13(b), insets) revealed sin-

gle- and poly-crystalline characteristics. In particular, the

FFT-SAED pattern of pPPy nanoparticles (Figure 13(a),

inset) revealed a clear diffraction spot of a single-crystalline

characteristic. From an atomic point of view, conventional

polymers and plasma-based polymers are composed of many

conjugated bonds without a constant direction. However, the

polymer that was polymerized using the proposed APPJs

with a bluff-body exhibited a single-crystalline characteristic

with a constant direction of atoms. As shown by the TEM

images in Figure 13, the directional arrangement of the poly-

mer atoms was likely caused by ion and electron-charged

particles with a constant directionality in the intense and

broadened plasma, resulting in a single-crystalline character-

istic for the pPPy nanoparticles. Furthermore, from a molec-

ular point of view, the single-crystalline characteristic of the

pPPy nanoparticles may also have been caused by the

regular-uniform and upright networks, as shown by the SEM

image in Figure 12 (case III). Investigations of these detailed

mechanisms will be reported in future work. Therefore, the

current experimental results revealed that single-crystalline

high-density plasma-polymerized pyrrole (pPPy) nanopar-

ticles were successfully synthesized at a low temperature

when using the advanced atmospheric pressure plasma poly-

merization technique. The nano-structures of the resulting

pPPy were visualized using energy dispersive x-ray spectros-

copy (EDS) elemental mapping and high-angle annular dark-

field scanning TEM (HAADF-STEM), as shown in Figures

13(c) and 13(d). Plus, the EDS and elemental mapping

results revealed that the pPPy nanoparticles were exclusively

composed of C, O, and N. Consequently, the TEM analysis

of Figure 13 indicated that the plasma polymerization using

the proposed APPJs in case III generated uniform pPPy

nanoparticles of small size with a single-crystalline

characteristic.

Figure 14 shows the Fourier transform infrared spectros-

copy (FTIR) spectra of the pPPy nanofibers and nanoparticle

thin film synthesized on the plastic substrates with the five

different cases (cases I–V in Figure 2(a)) using the advanced

APPJ technique. For all five cases, the deposition time was

identical, i.e., 60 min, with a low process temperature. The

FTIR spectra of these polymers films were measured using a

Perkin-Elmer Frontier spectrometer between 650 and

4000 cm�1.

As shown in Figure 14, the FTIR spectra were deter-

mined using a transmittance method since the pPPy films

grown in this experiment were too thin to use an absorbance

method. In cases III–V, the characteristic peaks of pPPy

were measured at 3285 cm�1 (N-H stretching with hydrogen

bonded 2� amino groups) and 2960 cm�1 (aliphatic C–H

stretching absorption). Plus, a –C¼H aliphatic vibration

FIG. 13. Transmission electron micros-

copy (TEM) images of pPPy nanopar-

ticles prepared via APPJs in case III.

High-resolution (HR) TEM images of

pPPy nanoparticles with (a) single-

crystalline and (b) poly-crystalline;

insets in (a) and (b) are fast Fourier

transform (FFT) of corresponding

selected area electron diffraction

(SAED) patterns of pPPy nanopar-

ticles. (c) High-angle annular dark-

field scanning TEM (HAADF-STEM)

and (d) energy dispersive x-ray spec-

troscopy (EDS) elemental mapping

images of O, C, and N. Scale bar¼ 2 nm

in (a) and (b), whereas scale bar

¼ 20 nm in (c) and (d).

023506-13 Kim et al. Phys. Plasmas 24, 023506 (2017)

peak was observed at 2215 cm�1. These peaks implied that

some of the pyrrole rings in the polymer were frag-

mented.28,31,71 Furthermore, different absorptions corre-

sponding to alkenes that reflected broken rings showed peaks

at 805 cm�1 and 730 cm�1, plus there was a C¼C double

bond peak at 1690 cm�1. These peak regions were evidence

of films with a better electric conductivity,28,31,72 while also

indicating the successful formation of pPPy. Consequently,

these experimental results confirmed that proper control of

the process parameters with the proposed APPJ device, that

is, case III, enabled the growth of high-density ultra-fast

pPPy thin films and nanoparticles at a low temperature. It is

also expected that the proposed device can be used for

plasma polymerization with various monomers. Moreover,

from a treatment and etching point of view, cases IV and V

can be beneficial due to direct contact of the substrate with

an intense plasma plume and strong streamer-like plasma.

Therefore, to reveal the plasma mechanism of the proposed

APPJs for large-area treatment, several options (e.g., the

number of jet arrays, bluff-body size) will be explored in

future studies.

IV. CONCLUSIONS

This paper investigated the use of a glass guide-tube and

polytetrafluoroethylene (PTFE) bluff-body as a main part of

the advanced atmospheric pressure plasma source for the

low-temperature synthesis of single-crystalline high density

plasma-polymerized pyrrole (pPPy) nano-materials, based

on parametric studies of the intense and broadened atmo-

spheric pressure plasma jets (APPJs). Three process parame-

ters, including the position of the bluff-body, Ar gas flow

rate, and remoteness of the substrate from the intense and

broadened plasma, were varied and the experimental and

numerical results confirmed that intense and broadened

plasma showing a glow-like discharge characteristic was

produced in a large area when properly adjusting the Ar gas

flow rate and position of the bluff-body with respect to the

guide-tube. In particular, the appropriate location of the

bluff-body with respect to the guide-tube under specific Ar

gas flow rates enabled full cracking of the monomers in the

nucleation region due to a longer sustainment of the Ar spe-

cies. Furthermore, positioning the substrate away from the

intense and broadened plasma (i.e., remote condition) facili-

tated successful low-temperature synthesis of single- and

poly-crystalline pPPy nanoparticles with a size of a few tens

of nanometers, due to the suppression of etching and thermal

damage on the glass or plastic substrate during the plasma

polymerization. Thus, by properly adjusting the three process

parameters, single-crystalline pPPy nanoparticles linked

together in uniform and upright networks were obtained

when using the proposed APPJ device, as confirmed by field

emission scanning electron microscopy (FE-SEM) and trans-

mission electron microscopy (TEM). It is also expected that

the growth of pPPy nanoparticles with a single-crystalline

property using a low-temperature process will provide a

unique advantage for many applications, such as molecular

electronics, opto-electronics, and nanodrug/gene delivery in

nanomedicine.

ACKNOWLEDGMENTS

This work was supported by a National Research

Foundation of Korea (NRF) grant funded by the Korea

Government (MOE) (No. 2016R1D1A1B03933162).

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