Atmospheric-pressure plasma jets: Effect of gas flow, active species, and snake-likebullet propagationS. Wu, Z. Wang, Q. Huang, X. Tan, X. Lu, and K. Ostrikov

Citation: Physics of Plasmas (1994-present) 20, 023503 (2013); doi: 10.1063/1.4791652 View online: http://dx.doi.org/10.1063/1.4791652 View Table of Contents: http://scitation.aip.org/content/aip/journal/pop/20/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Characteristics of atmospheric-pressure non-thermal N2 and N2/O2 gas mixture plasma jet J. Appl. Phys. 115, 033303 (2014); 10.1063/1.4862304 Investigation on streamers propagating into a helium jet in air at atmospheric pressure: Electrical and opticalemission analysis J. Appl. Phys. 114, 103304 (2013); 10.1063/1.4820570 Atmospheric pressure He-air plasma jet: Breakdown process and propagation phenomenon AIP Advances 3, 062117 (2013); 10.1063/1.4811464 Spectroscopic measurement of electric field in atmospheric-pressure plasma jet operating in bullet mode Appl. Phys. Lett. 99, 161502 (2011); 10.1063/1.3653474 A simple cold Ar plasma jet generated with a floating electrode at atmospheric pressure Appl. Phys. Lett. 93, 011503 (2008); 10.1063/1.2956411

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Atmospheric-pressure plasma jets: Effect of gas flow, active species, andsnake-like bullet propagation

S. Wu,1 Z. Wang,1 Q. Huang,1 X. Tan,1 X. Lu,1,a) and K. Ostrikov2,3,1

1State Key Laboratory of Advanced Electromagnetic Engineering and Technology,Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China2CSIRO Materials Science and Engineering, PO Box 218, Lindfield NSW 2070, Australia3School of Physics, The University of Sydney, Sydney NSW 2006, Australia

(Received 19 November 2012; accepted 22 January 2013; published online 11 February 2013)

Cold atmospheric-pressure plasma jets have recently attracted enormous interest owing to numerous

applications in plasma biology, health care, medicine, and nanotechnology. A dedicated study of the

interaction between the upstream and downstream plasma plumes revealed that the active species

(electrons, ions, excited OH, metastable Ar, and nitrogen-related species) generated by the upstream

plasma plume enhance the propagation of the downstream plasma plume. At gas flows exceeding

2 l/min, the downstream plasma plume is longer than the upstream plasma plume. Detailed plasma

diagnostics and discharge species analysis suggest that this effect is due to the electrons and ions that

are generated by the upstream plasma and flow into the downstream plume. This in turn leads to the

relatively higher electron density in the downstream plasma. Moreover, high-speed photography

reveals a highly unusual behavior of the plasma bullets, which propagate in snake-like motions, very

differently from the previous reports. This behavior is related to the hydrodynamic instability of the

gas flow, which results in non-uniform distributions of long-lifetime active species in the discharge

tube and of surface charges on the inner surface of the tube. VC 2013 American Institute of Physics.

[http://dx.doi.org/10.1063/1.4791652]

I. INTRODUCTION

Cold atmospheric pressure plasma jets (C-APPJs) have

recently attracted a lot of attention due to their numerous appli-

cations in plasma biology, health care, and medicine, as well

as surface and materials processing and nanotechnology.1–12

C-APPJs usually generate plasmas in surrounding air rather

than in confined space.13–16 Hence, they can be used for direct

treatment and there is no limitation on the size of the objects

that can be treated. Studies on the propagation mechanism of

C-APPJs reveal that the plasma jets are formed by fast-moving

plasma bullets rather than a continuous discharge glow as com-

monly perceived by a human eye.17–28

Further investigations on the dynamics of C-APPJs in

noble gases reveal that the plasma plumes propagate at a

speed of 104–105 m/s,17–23 which is several orders of magni-

tude higher than the gas flow velocity. This is why it is gener-

ally accepted that the propagation of the plasma plumes is

driven electrically rather than by the gas flow.

It was recently revealed that the gas flow rates affect the

length and the modes of C-APPJ operation.23,25 However, this

effect is rather indirect because diffusion of the surrounding

air into the working gas stream strongly affects the gas com-

position and eventually the propagation of the plasma plume

in open air.

To the best of our knowledge, there is presently no evi-

dence that the gas flow rate has a direct effect on the propaga-

tion of C-APPJs in noble gases. To clarify this issue, here we

report on a dedicated experimental study on the behavior of

two plasma plumes generated upstream and downstream the

gas flow in a quartz tube. The quartz tube is used to avoid the

effect of the diffusion of the surrounding air. It is found that

when the gas flow rate is higher than �2 l/min not only do the

lengths of both plasma jets shorten but an asymmetry devel-

ops such that the downstream plasma plume becomes unex-

pectedly longer than the upstream plasma plume. The higher

is the gas flow rate, the larger is the length difference between

the two plasma plumes. Besides, high-speed photography

reveals that the plasma plumes propagate in snake-like

motions, which is very different from the previous reports.

II. EXPERIMENTAL

The schematic of the experimental setup is shown in

Fig. 1(a). The high-voltage (HV) electrodes are made of two

stainless steel needles, which are inserted into the quartz

tube. The distance between the tips of the two needles of the

same size and shape is 77 mm. The radius of the needles is

about 100 lm. The inner diameter of the quartz tube is about

0.8 mm. The working gas flows into the quartz tube from the

right end. Research-grade Ar gas (99.999% purity) is used in

the experiment. To keep the ambient air contamination in the

discharge region as low as possible, another quartz tube with

a length of 2 m is connected to the left end of the quartz tube

as shown in Fig. 1(a).

Thus, the two plasma plumes are generated in the gas

with the same composition. In other words, the difference

between the gas composition in the two plasma plumes

caused by the air diffusion from the left end of the tube can

be neglected; this is also confirmed by the computational

fluid dynamics (CFD) modeling results obtained using FLUENT

software.29 Moreover, when the gas flow rate is higher thana)Electronic address: luxinpei@hotmail.com.

1070-664X/2013/20(2)/023503/7/$30.00 VC 2013 American Institute of Physics20, 023503-1

PHYSICS OF PLASMAS 20, 023503 (2013)

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1 l/min, the amount of air around the two plasma plumes is

even lower compared with the amount of impurities in

research-grade argon. Besides, the optical emission spectros-

copy measurements (presented later in this paper) show that

optical emission from nitrogen- or oxygen-related species is

extremely low when the Ar flow rate is higher than 1 l/min.

The two needles are connected to the high-voltage pulsed

dc power supply in parallel (amplitude up to 10 kV, repetition

rates up to 10 kHz, and pulse width variable from 200 ns to

dc). The applied voltage of 8 kV, pulse frequency of 9 kHz,

and pulse width of 400 ns were fixed in this series of experi-

ments. When HV pulsed dc voltage is applied to the two nee-

dles and working gas is injected into the quartz tube with a

gas flow rate of 2 l/min, the plasma plumes are generated

inside the quartz tube as shown in Fig. 1(b). It should be men-

tioned that all images in the paper are spectrally broadband.

III. RESULTS AND DISCUSSION

A. I-V waveforms of the upstream plasma plume

To study the I-V characteristics of the plasma plume, a

P6015 Tektronix HV probe is used to measure the voltage on

the needle, and a Pearson 2877 current probe is used to mea-

sure the plume current. The distance between the current

probe and the tip of the right needle is about 5 mm. The di-

ameter of the probe jaw is about 6.1 mm, which is larger

than the outer diameter of the tube (1.8 mm). It allows us to

measure the current of the plasma plume by letting the

plasma plume across the probe jaw as shown in Fig. 1(a).

Figure 2 shows the applied voltage and plume current wave-

forms of the upstream plasma. Two distinct discharge current

pulses per applied voltage pulse are observed. The mecha-

nism of the second discharge is related to the voltage

induced by the residual charges during the first discharge,

and more explanation can be found elsewhere.30 Moreover,

its peak current reaches as high as 280 mA, which means

that the plasma plumes are very active. However, it should

be mentioned that the current probe perturbs the plasma

plume during the current measurement.

B. Effect of gas flow rate on the length of the twoplasma plumes

When a high voltage is turned on and Ar flows through

the quartz tube, two plasma plumes are generated in front of

the two needles. Figure 3 shows the photographs of the

plasma plumes taken with a Sony digital camera (DSC-HX1)

at an exposure time of about 100 ms for different Ar flow

rates. With the increase of the Ar flow rate from 0.2 to

1 l/min, both the plasma plumes become longer. The differ-

ence between the lengths of the two plasma plumes is very

small when the Ar flow rate is lower than 0.5 l/min. When

the argon flow rate is increased to 2 l/min, the both plasma

plumes become shorter. Moreover, the length of the

upstream plasma plume Lup becomes even shorter than the

length of the downstream plasma plume Ldown. Figure 4

shows the difference between lengths Lup and Ldown. In the

following, a detailed analysis of the effect of the Ar flow rate

on the length of the plasma plumes is presented.

As the Ar flow rate is increased, the length of the plasma

plumes increases at first. This can be explained as follows.

Due to the diffusion of the ambient air, there are always

some trace amounts of air inside the quartz tube. When the

Ar flow rate is increased from 0.2 to 1 l/min, the concentra-

tion of air in the quartz tube is reduced. Thus, the lower is

the concentration of air in the background gas, the larger is

the length of the plasma plume. The CFD modeling shows

that at Ar flow rates above �1 l/min, the concentration of the

air in the tube becomes too low (�10�5 fraction of the Ar

flow) to affect the discharge behavior.

To explain why both the plasma plumes get shorter

when the flow rate is further increased, the Reynolds num-

bers of the Ar flow inside the quartz tube were calculated for

different gas flow rates. The calculated Reynolds number for

FIG. 1. (a) Schematic of the device and

(b) spectrally broadband photograph of

the discharge. The exposure time is

approximately 100 ms.

FIG. 2. The applied voltage and the plume current waveforms of the upstream

plasma versus time. The Ar flow rate is 3 l/min.

023503-2 Wu et al. Phys. Plasmas 20, 023503 (2013)

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a flow rate of 2 l/min is about 4568. Since the critical Reyn-

olds number is 2320 to distinguish between the laminar and

transition flows in the tube, the flow becomes hydrodynami-

cally unstable when the flow rate is 2 l/min or higher. Hence,

the higher is the flow rate, the stronger is the hydrodynamic

instability, which in turn shortens the plasma plume.

To elucidate why the upstream plume becomes shorter

than the downstream one with the increase of the gas flow

rate, first, we switched the positions of the two needles. The

result was exactly the same, which means that any small dif-

ference in the sizes and shapes of the two needles had no

influence on the observed phenomenon.

Second, for the upstream plasma plume, the gas flows

from the tube with the needle in the centre to the free space

where there is no needle. On the contrary, for the downstream

plasma plume, the gas flows from the free space to the place

where there is a needle. This could cause the flow mode

difference and might result in the observed phenomenon. To

verify if this is the actual reason why Lup is shorter than Ldown,

another experiment was carried out. As shown in Fig. 5, when

a single needle is used as an electrode, the gas flow direction

can either be the same as the plasma plume propagation direc-

tion or the opposite. Figure 5 shows that the plasma plume

with the gas flow direction which is the same as the plasma

plume propagation direction is longer than the plasma plume

when the plasma plume propagation direction and the gas

flow direction are the opposite. This trend is the opposite with

the results in Fig. 2, where Lup is smaller than Ldown. This phe-

nomena may be related to hydrodynamic effects.31,32

The other possibility is the interactions between the

upstream and downstream plasma plumes through the

exchange of active species. Indeed, since the gas flows from

the right (upstream) to the left (downstream), long-living

active species (e.g., electrons, ions, radicals, and excited spe-

cies) generated upstream may flow into the downstream

region of the tube. Thus, the background gas of the down-

stream region may have higher concentrations of the active

species. This could be the reason why Lup is smaller than

Ldown. In the following, further experiments are carried out

to investigate what kind of active species may flow from the

upstream to the downstream regions thereby affecting the

propagation of the plasma plume.

FIG. 3. Photographs of the plasma

plumes at different gas flow rates in Ar

working gas. The photographs are spec-

trally broadband. The exposure time is

approximately 100 ms.

FIG. 4. The length difference of the downstream and the upstream plasma

plumes versus the gas flow rate.

FIG. 5. Photographs of the discharges produced with a single needle electrode

in two gas flows with the opposite directions. The Ar flow rate is 4 l/min. The

photographs are spectrally broadband. The exposure time is approximately

100 ms.

023503-3 Wu et al. Phys. Plasmas 20, 023503 (2013)

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C. Effect of active species on the propagation of theplasma plume

In order to affect the propagation of the downstream

plasma plume, the active species have to effectively flow

from the upstream to the downstream region. For a flow rate

of 2 l/min, the time required for the active species to flow

from the upstream to the downstream region is about 0.5 ms.

Hence, the lifetimes of the active species that could affect

the propagation of the plasma plume should be �0.5 ms or

longer. Since the plasma-on time is only 400 ns, the active

species generated in the upstream plasma during the voltage

pulse could not affect the downstream plasma during the

same voltage pulse. However, if the lifetime of the active

species is longer than the pulse repetition time, the active

species generated by a voltage pulse in the upstream plasma

could flow to the downstream region and affect the propaga-

tion of the plume generated by the subsequent voltage

pulses.

To confirm this possibility, the pulse frequency was

increased from 2 to 10 kHz. It was found that both lengths

Lup and Ldown did not change. This means that the lifetimes

of the active species which may affect the propagation of the

plasma plume are either much longer than 0.5 ms or shorter

than 0.1 ms. The species with the lifetime shorter than 0.1 ms

do not have enough time to flow between the upstream and

the downstream regions. Therefore, the lifetimes of the rele-

vant active species are most likely longer than 0.5 ms.

1. Excited OH* and Arm* species

To identify the active species generated by the plasma

plumes, optical emission spectra of the two plasma plumes

were measured. The emission spectra were collected at a dis-

tance of 5 mm away from each needle in the upstream and

downstream regions. Figure 6 shows the emission spectra of

the two plasma plumes in the spectral range from 200 to

900 nm. The spectra clearly show that excited OH* and Ar*

exist in the plasma plumes. The intensity of the excited Ar*

in the downstream region is stronger than in the upstream

region. The OH emission likely originates from dissociation

of the H2O impurity of the Ar gas. The lifetime of excited

OH* is less than 1 ls;33 it is thus unlikely that this species

plays any significant role in the observed phenomenon.

The other potentially relevant species are Arm* metasta-

bles. To estimate their lifetimes, we have analyzed the rele-

vant loss processes of these species. The main loss process

of metastable Arm* at atmospheric pressure is the three-body

collision

Ar�m þ Ar þ Ar ¼ Ar�2 þ Ar

with the rate constant k¼ 1.4� 10�32 cm6 s�1.34–36 There-

fore, the characteristic decay time of the metastable Arm* can

be estimated to also be less than 1 ls. This species is quickly

converted to the excited Ar2* through three-body collisions.

Then, due to the short lifetime (less than 3.2 ls) of the

excited Ar2* species,37 it quickly quenches to the ground state

by emitting a photon before the following discharge pulse.

Hence, Arm* species are also not the cause of the observed

phenomenon.

2. Excited N2* species

As mentioned above, due to the very low presence of

the air in the discharge tube, excited N2* species are also not

expected to affect the interaction between the two plasma

plumes. To confirm this, 1% of N2 was added to the Ar gas

flow. Figure 7 depicts the propagation of the two plasma

plumes with and without nitrogen addition. Detailed analysis

shows that the difference between Lup and Ldown remains the

same for the two experimental conditions. Therefore,

nitrogen-related species also have no effect on the length dif-

ference between the upstream and downstream plumes.

3. Charged particles

The remaining active species that are likely to play a

role in the interactions between the two plasma plumes are

charged particles, namely electrons and ions. For the Ar

plasma plumes with the flow rate of more than 1 l/min, Arþ

ions are initially produced by electron impact. However, the

characteristic time of conversion of Arþ into Ar2þ through

three-body collisions involving Ar atoms

Arþ þ Ar þ Ar ¼ Arþ2 þ Ar

is less than 100 ns at atmospheric pressure given the rela-

tively high rate constant of this reaction k¼ 1.46–3.9

� 10�31 cm6 s�1.38 Therefore, the Ar2þ ions should be the main

positive ion in the plasma plumes. At atmospheric pressure, it

is reasonable to assume that the electron-ion recombination is

the dominant loss process of the Ar2þ ions. According to the

FIG. 6. Optical emission spectra of the upstream and downstream plasmas

in the spectral range from 200 to 900 nm.

FIG. 7. Photographs of the plasma plumes with the addition of nitrogen to

the Ar flow. The Ar flow rate is fixed at 4 l/min. The photographs are spec-

trally broadband. The exposure time is approximately 100 ms.

023503-4 Wu et al. Phys. Plasmas 20, 023503 (2013)

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measurement results,39–41 during the discharge, the electron

density of the plasma plumes could reach �1013 cm�3.

Therefore, assuming the initial densities of electrons ne0

and ions ni0 to be �1013 cm�3, the rate of variation of the

electron density can be expressed as

@ne

@t¼ �bn2

e ; (1)

where the electron-ion recombination coefficient b is �10�7

cm3/s.38 From Eq. (1), one obtains

neðtÞ ¼1

1

ne0

þ bt

for the temporal variation of the electron density. For the pulse

repetition frequency of 1 kHz, the electron density drops to

about �1010 cm�3 just before the next voltage pulse.

As mentioned above, it takes approximately 0.5 ms for

the gas to flow from the upstream to the downstream region

when the gas flow rate is 2 l/min. Hence, the electrons and

the ions generated by a voltage pulse in the upstream plasma

region could flow to the downstream region and, therefore,

affect the propagation of the downstream plasma plume gen-

erated by the subsequent voltage pulses.

D. Effect of surface charges

The surface charge accumulation plays an important role

in the plasma plume formation and dynamics, as confirmed by

other authors for a variety of atmospheric plasma configura-

tions.42–44 In our experiment, the (small) differences in the

bullet initiation processes between the downstream and

upstream jets may result in different lengths of the surface

charge areas on the inner surfaces of the tube. This difference

may affect the lengths of the plasma plumes. Because of the

limitations of the available equipment, we were not able to

directly measure these charge distributions. However, one can

speculate that such areas may be expected to be slightly larger

for the upstream plume than for the downstream plume when

only one “upstream” or “downstream” needle is used alone.

When the two needles are used, the surface charge distribution

on the tube surface between the needles may be fairly uniform

when the gas flow is slow.

One can also expect that at the place where there is no

plasma plume (around but not exactly at the mid-point

between the two needles), the surface charge density is low.

Moreover, the surface charges are mostly supplied from the

upstream plume. Indeed, the density of charged particles typ-

ically experiences a 3 orders of magnitude drop in about

1 ms. During this short period of time, the charges can travel

�1 cm in the gas flow with a typical velocity of �10 m/s.

When the gas flow becomes faster (and hence turbulent),

the non-uniform impingement of charged species onto the

inner surface of the tube very likely leads to non-uniform

charge distributions. This also affects the dynamics of the

plasma plumes, and in particular, the observed snake-like

behavior. Importantly, because the flow strongly affects the

charge distribution of the surface charges, it is very difficult

(if possible at all) to separate the effects of the gas flow and

the surface charges, as is not only evident from our experi-

ments but also from the results of other authors.42–45 Further

studies involving direct measurements of non-uniform

charge distributions are needed to clarify these important

issues.

E. Propagation dynamics of the plasma plumes

To further investigate the gas flow effect on the propaga-

tion of the plasma plumes, a fast intensified charge-coupled

device (ICCD) camera (Princeton Instruments, Model

PIMAX2) was used to capture the dynamics of the discharge.

Figure 8 shows the ICCD images of the plasma plumes. The

exposure time of the camera was fixed at 4 ns. Each image

represents a single-shot video frame. Figures 8(a)–8(h) cor-

respond to the primary discharge and (i)–(n) correspond to

the secondary discharge ignited during the fall time of the

voltage pulse. It is worth emphasizing that both the plasma

plumes propagate in a snake-like motion, which is very dif-

ferent from the previous reports.17–23

It is noteworthy that the downstream plasma initiates

about 8 ns earlier than the upstream plasma as shown in Figs.

8(a) and 8(b). Besides, Fig. 9 shows the propagation veloc-

ities of the two plasma plumes versus time. It clearly shows

that both the primary and the secondary downstream plasma

plumes have higher propagation velocities than the upstream

plasmas. These two phenomena could be due to the gas flow

effect, which results in the higher electron density in the

downstream plasma region, as discussed in Sec. III D. It

should be emphasized that the propagation velocity of the

plume reaches as high as 3� 105 m/s, which means the dis-

charge is streamer-like.20

Regarding the snake-like propagation behavior of the

plasma plumes, it takes place when the gas flow rate is higher

FIG. 8. ICCD single-shot images of the plasma plumes. The exposure time

is fixed at 4 ns. The time labeled on each image corresponds to the dc pulse

rise time in Fig. 2. The working gas is Ar and the gas flow rate is 5 l/min.

The images are spectrally broadband.

023503-5 Wu et al. Phys. Plasmas 20, 023503 (2013)

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than 2 l/min. As discussed above, under such conditions:

(i) the effect of air diffusion from the open end of the tube is

negligible; (ii) the gas flow is turbulent. Over many pulse

discharge cycles, the argon gas is not “pure” anymore in the

discharge region. Besides Ar (ground state), some long rela-

tive lifetime species such as residual charges exist in the back-

ground gas. For example, as mentioned in Sec. III B, the

density of residual charges just before the next voltage pulse

could be as high as �1010 cm�3 for the initial density of the

charge of �1013 cm�3. Therefore, the density of residual

charges is relatively high just before the next voltage pulse.

Within the timescale of the charge decay phase (the time

between consecutive pulses, which is �1 ms), the turbulence

fluctuations may significantly redistribute the residual charges

just before the next voltage pulse. This non-uniformity of the

residual charges may play a significant role in the formation

of the snake-like bullet patterns.

According to the analysis above, one can conclude that

because of the turbulence fluctuations, the densities of the

long-lifetime species such as charges are not uniform within

the tube and on the inner surface of the tube just before

the next voltage pulse. When the discharge is initiated, the

plasma plume propagates towards the place where (1) the

electric field is high (part of it is due to the non-uniform dis-

tribution of surface charges) and (2) the concentration of

background reactive species (such as charged particles) is

high. These factors contribute to the observed snake-like

propagation behavior. We emphasize that if the gas flow had

no direct effect on the propagation of the plasma plume, the

plasma would propagate like in a laminar gas flow. In other

words, the plume propagation would be uniform and stream-

lined, as in the previous reports.17–23

IV. CONCLUSION

The effects of the gas flow on the propagation of atmos-

pheric pressure plasma plumes have been studied. Within the

range of the Ar flow rates from 2 to 6 l/min, the length of the

downstream plasma plume was found surprisingly larger

compared to the upstream plume. Moreover, the length dif-

ference Ldown � Lup became larger with the increase of the

gas flow rate. Detailed investigations show that this phenom-

enon is due to the effects of the electrons and ions (rather

than excited OH, Ar metastables, or nitrogen-related species)

generated by the upstream plasma, which flow from the

upstream to the downstream regions thereby increasing the

background density of the charged species in the down-

stream plasma region. Besides, the ICCD images revealed

the highly unusual and previously unreported snake-like

propagation of the plasma plumes at gas flow rates above

�2 l/min. The-snake like plasma bullet behavior is probably

due to (1) the non-uniform distribution of the surface charges

on the inner surface of the tube and (2) the non-uniform dis-

tribution of active species such as charged particles inside

the tube. Both these effects are affected by the turbulent flow

of the gas.

A detailed analysis involving CFD modeling showed that

this kind of propagation was caused by the hydrodynamic

instability of the Ar flow. It was also shown that the down-

stream plasma plume is ignited earlier and propagates faster

than the upstream plume. This is also probably due to the

higher density of the charge species in the downstream gas

flow. These results have thus clarified the issue of the role of

the gas flow and active discharge species in atmospheric pres-

sure plasma jets and can be used in improving control of this

type of non-equilibrium plasmas in plasma biology, health

care, medicine, and nanotechnology applications.

ACKNOWLEDGMENTS

This work was partially supported by the National Natu-

ral Science Foundation (Grant Nos. 51077063 and 51277087),

Research Fund for the Doctoral Program of Higher Education

of China (20100142110005), and Chang Jiang Scholars Pro-

gram, Ministry of Education, People’s Republic of China, as

well as the Australian Research Council and CSIRO’s OCE

Science Leadership Scheme.

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