Characteristics of graphene grown through low power capacitive coupled radio frequency plasma enhanced chemical vapor deposition

Journal Pre-proof Characteristics of graphene grown through low power capacitive coupled radio frequency plasma enhanced chemical vapor deposition Yu-Chen Chang, Chun-Chieh Yen, Hung-Chieh Tsai, Tsung Cheng Chen, Chia-Ming Yang, Chia-Hao Chen, Wei-Yen Woon PII:

S0008-6223(19)31340-5

DOI:

https://doi.org/10.1016/j.carbon.2019.12.093

Reference:

CARBON 14929

To appear in:

Carbon

Received Date: 13 November 2019 Revised Date:

21 December 2019

Accepted Date: 30 December 2019

Please cite this article as: Y.-C. Chang, C.-C. Yen, H.-C. Tsai, T.C. Chen, C.-M. Yang, C.-H. Chen, W.-Y. Woon, Characteristics of graphene grown through low power capacitive coupled radio frequency plasma enhanced chemical vapor deposition, Carbon (2020), doi: https://doi.org/10.1016/ j.carbon.2019.12.093. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Characteristics of Graphene Grown Through Low Power Capacitive Coupled Radio Frequency Plasma Enhanced Chemical Vapor Deposition Yu-Chen Chang1, Chun-Chieh Yen1, Hung-Chieh Tsai1, Tsung Cheng Chen2, Chia-Ming Yang2,3,4,5, Chia-Hao Chen6, Wei-Yen Woon1* 1

Department of Physics, National Central University, Jungli 32054, Taiwan

2

Department of Electronic Engineering, Chang Gung University, Taoyuan, Taiwan

3

Institute of Electro-Optical Engineering, Chang Gung University, Taoyuan 333,

Taiwan 4

Biosensor Group, Biomedical Engineering Research Center, Chang Gung University,

Taoyuan, Taiwan 5

Department of General Surgery, Chang Gung Memorial Hospital, Linkou, Taiwan

6

National Synchrotron Radiation Research Center, Hsinchu, 30076, Taiwan, Republic

of China

Abstract We study the characteristics of graphene grown through low power capacitive coupled radio frequency plasma enhanced chemical vapor deposition (PECVD) and explore its application. Fully-covered, mostly single-layered, and highly defective graphene films 1

were

grown

on

Cu

substrates

through

plasma

composed

of

various

argon/methane/hydrogen gas ratio with low RF power within a minute under a moderate substrate temperature at 850℃. The structural, chemical, and electrical properties of the graphene films were measured through Raman spectroscopy, X-ray photoelectron spectroscopy, and Hall measurement, respectively. The Raman signatures were strongly affected by the argon/hydrogen flow rates and the subsequent thermal annealing, and is strongly correlated to carrier mobility of the film. Through analysis of the defect related Raman bands within the theoretical framework of activated defect model, it is concluded that the defects found in the graphene films are originated from ion bombardment during the growth processes. The vacancy defects on the graphene film provides adsorption sites for gaseous molecules and induces changes of charge transfer in the graphene film. The response of the graphene-based sensor fabricated with the PECVD grown graphene is 6 % under 100 ppb of which can be a candidate of potential

gas sensing material in the future.

Corresponding author email: [email protected] 2

,

1. Introduction Graphene has attracted tremendous interest in recent decade due to its unique opto-electronic, thermal, and mechanical properties [1–4]. So far, chemical vapor deposition (CVD) is the choice method to produce high quality graphene in large scale. However, the high thermal budget (including the high temperature and long process duration) for conventional CVD process is non-ideal to industrialization. Plasma enhanced chemical vapor deposition (PECVD) provides an alternative solution for the above issue. The radicals produced in a plasma can vastly lower the energy barriers for nucleation and growth process in CVD, thereby lower the overall thermal budget through shorten the growth time and lower the growth temperature [5– 7]. Previously, we have successfully built a custom made capacitive coupled PECVD (CC-PECVD) chamber that is able to grow fully covered graphene on copper substrate under low RF power within a minute. We have investigated the nucleation and growth dynamics of the grown graphene film and developed a modified Johnson-Mehl-Avrami-Kolmogorov model to describe the growth process [8]. Typically, in a RF-PECVD process, inert gas such as Ar is usually employed as the background gas where the discharge is ignited by electron impact through a RF electric field. Injecting a hydrocarbon precursor such as leads to excitation and dissociation of

into the Ar plasma

. For Ar, the interaction between electrons

and Ar atoms are dominated by elastic collisions due to its relatively high excitation (first excitation energy at 11.55 eV) and ionization energy (15.76 eV) [9,10]. As a result, in a

/

gas mixture, Ar plays the roles to enhance the dissociation

process, while maintaining the plasma stability in a PECVD process. Upon adsorbing on a thermally activated catalytic substrate such as Cu,

may be further

dissociated into C2 dimers or C atoms through a series of dehydrogenation processes. 3

Furthermore, adding rate since

into the gas mixture could promote the

dissociation

could serve as activator of the surface-bound carbon sources. However,

also serves as an etchant that could etch away the small graphene grains grown on Cu through forming volatile

at the grain edges [11]. The rich interactions

between the gas mixture component could greatly affect the nucleation and growth dynamics and the eventual properties of the grown film. Nevertheless, in a capacitive coupled plasma, due to the voltage-drop across the sheath layer developed between the glow discharge and substrate, ion bombardment of the graphene film is inevitable. The ion bombardment leads to defect generation in the graphene film, as evidenced by the high D-band measured in Raman spectra. The defects are generally undesirable for applications such as high mobility devices. However, the defects may become advantageous in other applications that require enhanced surface adsorption and reactivity. On the other hand, although plasma has been used in graphene synthesis for years [5,6,8,12–14], and in most cases defects are found in the grown graphene film, the origin of defects and their interactions with various plasma parameters are rarely explored in detail. In this work, we investigate the characteristics of the PECVD graphene films and relate the structural, chemical, and electrical properties to growth conditions, particularly on Ar flow rate and the CH /

ratio. Through understanding detailed mechanism of the defect generation,

we showed that controllability of the desirable quality of graphene film suitable for certain applications can be tailor-made. The gas sensing response of this fabricated graphene was investigated with the concentration of

down to the ppb level at

room temperature. 2. Experiment section The PECVD chamber was composed of a stainless-steel hollow oval electrode with a radio frequency (RF, 13.56 MHz) power supply (ULVAC RFS-02CA). Four 0.1 4

Tesla permanent magnet bars aligned parallel outside the chamber were implemented for better plasma confinement. The detailed growth process and the schematic plot of the PECVD setup can be found in supplemental materials (Fig. S1). In brief, Cu foil (elight, 99.9%, 25um) was electro-polished and pre-cleaned prior to the growth process. During the preheat process, Cu foil was heated to the target temperature (850℃) under a common Ar gas flow rate of 50 sccm. The Ar flow rate was then subsequently adjusted to the target flow rate (10 − 500 sccm) and the Ar plasma was

ignited by RF power (50W), followed by injection of CH /

gas mixture through a

shower head. The growth period is then defined by the length of CH /

injection

time while the RF plasma is turned on. In this work, the growth time was fixed at 1 min and the flow rate of

was fixed at 5 sccm. Subsequently, the RF power was

turned off and the sample was cooled down to room temperature by turning off the heating stage. The as-grown graphene films were then transferred onto

/

or

glass substrates through a bubble method [15] for further characterizations. The graphene films were examined through scanning electron microscopy (SEM) (Jeol JSM7000F at 10 KeV) and optical microscopy (OM) (BX51, Olympus). The topography information was acquired through atomic force microscopy (AFM). The crystallinity and the structural information were acquired through a homemade micro-Raman spectroscopy (μ-Raman) (532 nm laser, spatial resolution ~ 1.0 μm, spectral resolution ~ 0.5

!"

). The chemical bonding information were measured

through X-ray photoelectron spectroscopy (μ-XPS) (beam line 09A1 of the National Synchrotron Radiation Research Center, Hsinchu, Taiwan). The electrical properties were evaluated through Hall measurement by Van Der Pauw 4-points probe (ACCENT HL5500, magnetic field was 0.484 Tesla) and fabricated into a back-gated field-effect-transistor (FET) device. For Hall measurement, four indium (In) balls were placed on the graphene/

/

as electrodes. Additionally, to fabricate a 5

graphene-based back-gated FET, separated Au(30nm)/Ti(3nm) electrodes were /

plated on graphene/

through E-gun evaporation with a shadow mask. The

channel lengths and widths were approximate 50 μm . The measurement was performed at room temperature in ambient through a KEITHLEY 4200-SCS semiconductor characterization system. Graphene is suitable for the application of gas sensor due to its high surface area ratio, especially operated at room temperature. For fabrication of PECVD graphene-based gas sensor, the graphene film was transferred (10* )/ +(150* ) interdigitated electrodes (IDEs)

onto glass substrates with

at a spacing of 100 μm. In order to obtain the accurate concentration of injected gas, the flow rate of air as carrier gas was controlled to dilute the

,

or

permeation tube in the gas standards generator (FlexStream™ Base Module, KIN-TEK Laboratories Inc, USA) [16]. Different concentration of

or

,

gas

were introduced into the chamber for gas sensing tests. Then the mechanical pump was used to evacuate the residual gas for the recovery test. 3. Result & discussion Continuous graphene film can be grown on Cu under various growth conditions. Here we show a typical AFM image of a graphene film grown under an optimized /

growth condition (Ar/ 850℃) transferred on

/

= 180: 5: 5, RF power = 50 W, growth temperature = in Fig. 1(a). The cross section across the edge of the

transferred film is shown in the inset of Fig. 1(a). The average height measured is about 0.86 nm, typically value for a single layered graphene film measured in ambient AFM. The average Raman spectrum of this particular PECVD grown graphene is shown in Fig. 1(b). The spectrum showed four most prominent bands, including the D-band at ~1350

2D-band at ~2700

!"

, G-band at ~1580

!"

!"

, D’-band at ~1620

!"

, and

. The Raman mapping of 2 3 /24 , 23 /24 , 235 /24 , and full

width at half maximum (FWHM) of the 2D-band were shown in Figs. 1(c) to 1(f), and 6

Fig. 1. (a) AFM image of a graphene film transferred on / . The inset is the cross section across the edge of the transferred film. (b) The average Raman Fig. 2. (a) Raman spectra ofPECVD graphenegrown undergraphene. different Ar flow (b) The 2 3 /2of spectrum of this particular (c-f) Therate. Raman mapping 4 and 2D-band FWHM Ar flow rate, displaying four-regime behavior. (c) 2 3 /2 /24 , and 2D-band FWHM. aThe histogram of each 4 , 23 /2 4 , 235versus The 235 /24 versus flowinrate the regime corresponds to (b)). (d) The measurement wereAr shown the(colors inset ofofrespective figure. 2 3 /24 versus 235 /24 under different Ar flow rate, showing the “defect trajectory”. the histogram of each measurement were shown in the inset of respective figure. We first investigate the role played by Ar flow rate in the PECVD process. As shown in Fig 2(a). The Raman spectra were normalized with the intensity of the peak at ~950

!"

. Evidently, for all runs (

was fixed at 5 sccm), high defect

related D and D’-bands can be found. On the other hand, the intensity and 2D-band FWHM change significantly under different Ar flow rate. The 2 3 /24 and 2D-band FWHM versus Ar flow rate are shown in Fig 2(b), displaying a four-regime behavior. (I7) In the first regime, 2 3 /24 increases and 2D-band FWHM decreases as Ar flow rate increases from 10 to 100 sccm. Under conditions with very low Ar flow rate (10~30 sccm), only amorphous-like carbon film with low 2 3 /24 and high 2D-band FWHM can be found. Maximum 2 3 /24 8 1.8 can be found under Ar flow rate at 7

100 sccm, indicating single layered graphene is formed (2 3 /24 < 2 because of the presence of D-band). The above observations are attributed to generation of more abundant

radicals due to enhanced collision rate between the electrons from precursors. (II) In the second regime, 2 3 /24 remains at

excited Ar atoms and

~ 1.8 and 2D-band FWHM remains at ~ 40

!"

with Ar flow rate ranging from 100

sccm to 240 sccm. The wide window with respect to Ar flow rate condition suggested high reproducibility of high quality single layered graphene for this PECVD process. The above observation also suggested that the electron impact effect from excited Ar is saturated under a fixed RF power. (III) In the third regime, the process become less stable, 2 3 /24 and 2D-band FWHM showed large error bars (sample to sample) and less uniform (within sample) as Ar flow rate increases from 250 sccm to 280 sccm. (IV) In the fourth regime, whereas Ar flow rate increases from 280 to 500 sccm, 2 3 /24 decreases to 0.25 and 2D-band FWHM increases to 100

!"

. The above

observations are attributed to shortening of electron mean free path (MFP) in the plasma due the increased collision with the confluent Ar gas background (mostly neutral). The weaken electron impact effect results in less dissociated therefore less

radicals. Furthermore, the effect of

:

, and

ion-bombardment is

enhanced as Ar flow rate increases, as evidenced by the increased vacancy like defect density found in a plasma treated defect free graphene under similar Ar flow rate (Fig. S2). The enhanced

:

ion bombardment leads to more defect generation in the

PECVD grown graphene so that the 2D crystallinity is lost. 23 /24 has been used for estimating the defect density in graphene in many previous works [17–20], but it is found to be a less sensitive measurement in our case (see Fig. S3 in supplementary materials). Alternatively, the intensity ratio of the D’-band to G-band (235 /24 ) is found to be a more sensitive indicator. Eckmann et al.

suggested that 235 /24 is higher for vacancies than ;<, sites for the same defect 8

concentration (23 /24 ) [20]. The plot of 235 /24 versus Ar flow rate is shown in Fig

2(c). It shows that 235 /24 decreases as Ar flow rate increases in the first regime and

then increases in the second and third regime. Finally, 235 /24 saturates to 0.75 in the

fourth regime. Additionally, a theoretical prediction of 235 /24 versus different defect

length (=3 ) is shown in the equation (1) :[17,20]

(1)

>( ?35) >(4)

=

(BC D !BE D )

A (B D ! B D ) FG C E

HIJE D KL D

−G

HIMJC D HJE D N KL D

O+

Q [1

−G

HIJE D KL D

]

Within the activated defect model [17], the intensity of any defect activated peak I(x), where x = D or D′, corresponding to this equation [20]. Parameters adopted from literature were used: Also, the parameters

Q,35

A,3 Y

= 2.6 * ,

Q,35

= 1.4 * , and

A,35

= 0.45 [20].

= 0.5 and 0.75 for vacancies and ;<, -like defect were

used, respectively. Eckmann et al. suggested that the values 0.33 for pure ;<, sites

and 0.82 for pure vacancies [20]. It shows that the type of defect in the first regime is the combination of ;<, and vacancies. In fact, there is still a weak ion-bombardment under the conditions of low Ar flow rate. At the second to the fourth regime, the defect type is dominated by vacancy, which is caused by serious

:

ion-bombardment. The value used in the prediction is extremely closed to 0.82. The used values of the theoretical prediction are shown in Figs. S4(a) and (b) in supplementary materials, corresponding to the left and right side of Fig 2(c), respectively. To briefly conclude the effect of Ar flow rate in the PECVD process, we plot the “defect trajectory”. Fig 2(d) shows the plot of 2 3 /24 versus 235 /24 under

different Ar flow rate. In the first regime, 235 /24 decreases and 2 3 /24 increases as

Ar flow rate increases. The defect type is first dominated by ;<, and then becomes a

combination of ;<, and vacancies as Ar flow rate increases. From the second to the 9

Fig. 3. (a) Raman spectra of graphene under different

flow rate. (b) The

2 3 /24 and 2D-band FWHM versus flow rate, displaying a three-stage flow rate (colors of stage corresponds to (b)). behavior. (c) The 23 /24 versus

fourth regime, 235 /24 increases and 2 3 /24 decreases as Ar flow rate increases. The :

defect type is dominated by vacancies due to strong

ion-bombardment. The

above observation suggests that the defect type changes from sp3/vacancy mixture to vacancy dominated as Ar flow rate increases. Next we investigate the effect of

flow rate to the properties of graphene

grown through PECVD. Raman spectra acquired from graphene grown under different

flow rate (Ar was fixed at 180 sccm) were plotted in Fig. 3(a). The

Raman spectra were normalized with the intensity of the The plot of 2 3 /24 and 2D-band FWHM versus different

peak at ~950

!"

.

flow rate is shown in

Fig 3(b), displaying a three-stage behavior. (I) In the first stage, 2 3 /24 increases as

flow rate increases from 0 to 5 sccm and saturates to 2 3 /24 8 2 in the range of

5 to 15 sccm. The above observation suggests that hydrogen atom (H, dissociated from

) can help dehydrogenation of

(

+

↔

be regarded as a co-catalyst. In this stage, the partial pressure of

!"

+

) [11] and can is much less than

Ar. Hence, the plasma is dominated by Ar ions and energetic electrons. The mechanism of dissociation of

is dominated by the impact of energetic electrons,

with a little help of dehydrogenation effect from hydrogen, i.e., “physical reaction” is dominant under low

flow rate. 10

(II) In the second stage, 2 3 /24 dramatically decreases to 0.3 as

flow rate

increases from 15 to 30 sccm. The probable cause of the above may be due to dilution effect of H2 in the Ar plasma under a fixed RF power. Mohanta et al. suggested that both electron density and temperature were reduced with the admixing of

in Ar

plasma by optical emission spectroscopy (OES) measurement [21]. It was found that the dominant

[ (656 *

) line along with the

\ (486 *

) transition and Ar I

transitions above 690 nm were reduced in the emission spectrum of Ar/

plasma

[21]. The above scenario can be understood by considering the following reactions:[21,22]

:

(2)

:

(3)

The addition of

+

→

:

+ G! →

+

+

∗

reduces the degree of ionization of the Ar plasma, resulting

in neutral Ar gas and excited hydrogen atoms (

∗

) by combining reactions (2) and (3).

As a result, the energy of electrons strongly decreases yet the excited H* is not enough to efficiently dehydrogenate

. Furthermore, the gaseous phase H atoms can

combine with the H atoms adsorbed on the surface of the chamber to form the vibrationally excited

∗

molecules (

), after atomic hydrogen adsorbing on the

surface of chamber. Therefore, the following reactions can occur between hydrogen ions (

:

and

):[21,22]

(4) (5)

The produced

∗

:

:

+

:

∗

→

+ G! →

:

+

+

∗

can further dissociate to produce H and 11

∗

, according to the

reaction (5). The reduction of density of electrons can be attributed to both of reactions (3) and (5). Also, the reduction of temperature of electrons can be attributed ∗

to reentry flow of

into the expanding plasma for the fast ionization loss in

plasma [21,22]. The combine effects result in incomplete dehydrogenation of and lead to deposition of amorphous carbon film on the Cu surface. Therefore, the second stage can be regarded as a transition stage between electrons impact (physical reaction) dominant stage and dehydrogenation effect of hydrogen (chemical reaction) dominant stage. (III) In the third stage, 2 3 /24 gradually increases as

flow rate increases. In

particular, 2 3 /24 8 2 can be obtained under the condition of sccm. In this stage, the partial pressure of

flow rate = 50

is comparable to the partial pressure

from Ar. Therefore, a large number of excited hydrogen atoms can be found in the plasma, providing enough ability for dehydrogenation of reaction (

+

↔

!"

+

) [11]. Nevertheless, it would take much longer

time to grow fully covered graphene film under high the enhanced etching effect of

through chemical

flow rate condition due to

(xH + graphene ↔ (graphene − C) +

Graphene nucleation density at high

) [11].

flow rate is reduced and the graphene grain

size is enlarged, as shown in our previous work [8]. Overall speaking, the above observations show the competition of energetic electrons impact (physical reaction) and dehydrogenation effect of hydrogen (chemical reaction) in a capacitive coupled RF plasma. The plot of 23 /24 versus

23 /24 decreases as

flow rate is shown in Fig 3(c). It is obviously that

flow rate increases, excepting for the deposition of

amorphous carbon in the second stage. It is attributed to the following two reasons. First, the graphene grain size (=d ) becomes bigger as the etching effect of

flow rate increases, due to

(slow growth rate) [11]. According to the Tuinstra-Koenig 12

Fig. 4. The Hall mobility versus (a) 2 3 /24 and (b) 23 /24 . (c) The sheet resistance is correlated to both of 2 3 /24 and 23 /24 . Low sheet resistance

corresponds to high 2 3 /24 and low 23 /24 samples, or vice versa.

(TK) relation, 23 /24 is inversely proportional to the grain size : 23 /24 ∝ 1/=d [23]. Under low

flow rate, the small grains grown at the manifold nucleation sites on

the Cu surface were not etch effectively, and the incoming carbon sources are distributed among those grains during growth, resulting in a high graphene nucleation density and a small graphene grain size. Under high

flow rate, etching effect of

reduced the nucleation sites, slows down the graphene nucleation rate, whereas the hydrogen termination at the grain edge slow down the growth rate. Therefore, low graphene nucleation density and big graphene grain can be achieved at a high flow rate with the same growth time. Second, the ion-bombardment effect may be reduced under a high

flow rate. Previous literature suggested that the diffusion of

hydrogen is fast in hydrogen containing plasma and atomic hydrogen has large adsorption rate on metal surfaces [21,24]. Hence, the adsorption of H atoms produced in Ar/

plasma can take place on the walls of the reactor. Under low

condition, the concentration of

:

flow rate

in the plasma is high and the plasma sheath is

thick, resulting in serious ion-bombardment. As a consequence, 23 /24 is high under low

flow rate. As

flow rate increases, the number of

:

become fewer and

the color of the plasma becomes purple (see supplementary materials for the color changes as plasma condition changes in Fig. S5), corresponding to the combination of 13

the reactions (2) and (3). Importantly, the ignited regions of the plasma only occur near the chamber surface, corresponding to the previous results [21,24]. Therefore, the ion-bombardment is reduced due to the fewer plasma sheath under high

:

in the plasma and the thinner

flow rate condition, resulting in lower 23 /24 .

The electrical properties of the PECVD grown graphene is examined through Hall measurement. It is found that the mobility is proportional to 2 3 /24 but not

correlated to defect density (23 /24 ), as shown in Figs. 4(a) and 4(b). In contrast, the sheet resistance is correlated to both of 2 3 /24 and 23 /24 , as shown in Fig 4(c). Low sheet resistance corresponds to high 2 3 /24 or low 23 /24 samples, or vice versa.

From the centimeter scale measurement, it is found that the sheet resistance of as-grown PECVD graphene film can be as low as 4.8k Ω/sq and Hall mobility can be as high as 150

i !" ; !".

Fig. 5. (a) The Raman spectra of the thermally treated PECVD grown graphene

samples at different temperature. (b) The plot of 23 /24 versus thermal healing temperature. XPS spectra of the (c) as-grown (d) after 200℃ annealing (e) after 800℃ annealing samples. (f) Percentage of each bonding for different as-annealing samples. (g) Correlation between the frequencies of the G-band and the 2D-band of graphene (j4 , j 3 ). The orange and purple line represent the 14 = 0.7), respectively. strain (slope = 2.2) and P-type doping (slope

The structural defects (vacancies) in PECVD graphene which damaged by

:

ion-bombardment can be partially healed by thermal annealing. In order to study the effect of thermal healing on defect density (23 /24 ), the PECVD graphene transferred on

/

was heated to a target temperature under a low vacuum condition of 1

mTorr for 30 min in the same PECVD chamber. The Raman spectra of the thermally healed PECVD grown graphene samples are shown in Fig 5(a). The plot of 23 /24 versus thermal healing temperature is shown in Fig 5(b). The result shows that 23 /24 monotonically decreases as the thermal healing temperature increases. XPS measurement further shows that the concentration of ;<, (C-C) decreases and ;< (C=C) increases as annealing temperature increases, respectively, as shown in Figs. 5(c)-5(f). As-grown PECVD graphene sample shows 53.44% C=C bonds and 46.56% C-C bonds. After annealing at 200℃, C=C bonds increase to 59.36% and C-C bonds

decrease to 25.86%. After annealing at 800℃, C=C bonds increase to 80.73% and C-C bonds decrease to 9.95%. The above results may be attributed to annihilation of vacancies through incorporation of misplaced carbon atoms with the assistance of thermal energy input [25,26]. Another notable change in the Raman spectra is the broadening of D and G-band after the thermal treatment. It has been attributed to reduction of PMMA residue and the appearance of amorphous carbon on the graphene film [27]. Therefore, the observed structural and chemical changes from Raman and XPS data may be a combination of above factors. A closer look at the Raman spectra shows that the G-band and 2D-band exhibit blue shift after the thermal treatments. It is attributed to additional doping in graphene after annealing. The doping is likely caused by molecular adsorption on graphene (charge transfer between the molecules and graphene) [28]. Previous literature suggested that the increase of charge carrier concentration in graphene can be induced by adsorbed gas molecules (

,,

,

15

,

) [28,29]. Among them, adsorbed

and

molecules work as acceptors, whereas

,

and

work as donors.

After annealing at a high temperature, the residue left from transfer process (e.g., PMMA) on graphene are greatly reduced, which provide more adsorption sites for and

molecules. The

and

molecules in air can adsorb on graphene

and cause doping in graphene [29]. The carrier concentration 1 l 10"

!

to 3 l 10",

!

k

increases from

, also measured by Van Der Pauw 4-points probe,

confirming hole doping in as-annealing PECVD graphene. Additionally, correlation between the frequencies of the G-band and the 2D-band of graphene (j4 , j 3 ) can be used to distinguish the doping and strain effect in the graphene [30,31]. Fig 5(g) shows that the Raman peaks measured from the thermally treated graphene fall closely to the p-type doping line (slope = 0.7), re-confirming the conclusion of hole doping. The above observation suggests the PECVD grown graphene possesses the potential to be a good material for gas sensor.

Fig. 6. (a) Drain current (2n ) versus gate voltage (io ). The drain voltage was fixed in 0.1V. The inset is an optical photograph of Au/Ti electrodes deposited on the graphene which transferred onto Si

/Si. The scale bar is 250μm. (b) Drain

current (2n ) versus drain voltage (in ) with different gate voltage (io ) corresponds to the measurement in (a). 16

Furthermore, a back-gated field effect transistor (FET) structure is fabricated using one of the thermally healed PECVD grown graphene. The optical photograph of the FET is also shown in the inset of Fig 6(a). The typical 2n − io curve shows

shifted charge neutral Dirac point (i3qBdr ) at ~50V, indicating that there is some extrinsic hole-doping in the thermally healed PECVD graphene (main figure in Fig. 6(a)). The FET electron mobility is approximate to be 267.2 x

" n>} , { |} n|{

calculated by the equation, tuvw = yz and width of the device, and

o

i !" ; !" , as

where L and W are channel length

is the gate capacitance. Fig 6(b) shows the typical

2n − in curve with different io corresponds to the measurement in Fig 6(a). The electrical characteristics as measured from the FET structure indicates that the PECVD grown graphene is suitable for application for electrical applications. is known as one of the most common air pollutants in the environment. The detection of

in ppb level is a crucial issue, because even ppb level of

is

already sufficient to damage the human respiratory system. In addition, the adsorption (-3.04 eV) is much lower than CO (-2.33 eV) and

energy of

,

(-0.24 eV)

on defective graphene through the DFT calculations [32]. Therefore, adsorption of on the defective graphene is stable and is thus a good candidate for demonstrating the capability of our PECVD grown graphene as materials for gas sensing. The dynamic response of the resistance under different concentrations of and

,

diluted in dry air were measured in series. The schematic diagram

and optical photograph of PECVD graphene-based gas sensor is shown in Fig. 7(a). The

concentrations were 50, 100, and 150 ppb in a series and the

,

concentrations were 1, 3, and 5 ppm in a series of injection of the gas. A mechanical pump was used to pump out the test gas for the recovery step. The sensor response is defined as the relative change of resistance by exposure to a given gas at a certain 17

concentration: [16]

(6) Response (%) =

∆• •‚

l 100

where ∆ƒ is the change of resistance after exposure to a given gas and ƒq is the initial resistance. Fig. 7(b) shows the Raman spectra of two PECVD grown graphene samples used for the fabrication of gas sensor. The “PECVD 1” refers to an amorphous-like carbon film grown under Ar/

/

= 10:5:5 gas mixture. The

“PECVD 2” refers to a high-quality graphene grown under Ar/

/

= 140:5:5

gas mixture. Fig. 7(c) shows the dynamic response versus time for the

sensor.

“PECVD 1” sensor is insensitive because the amorphous carbon has poor electrical transport property and can block the adsorption of injection of 100 ppb

[16]. On the contrary,

to the “PECVD 2” sensor reduces the resistance quickly

with a 6% change, as shown in Fig. 7(d). It is a remarkable sensitivity for graphene-based

gas sensing due to the abundance of vacancies in PECVD

grown graphene. The vacancy defects on the graphene film provides adsorption sites for gaseous molecules and induces changes of charge transfer in the graphene film. Furthermore,

acts as electron acceptor [33], transferring electrons from the

graphene film, followed by making graphene become more p-type doping and more conductive. Regarding the poor sensor recovery behavior, it is attributed to the strong adsorption energy of

on defective graphene („dn = −3.04 Gi ) [32,34]. It

makes the coverage of adsorbed

on the graphene surface is still high and the

response saturates as the concentration increases. Fig. 7(e) shows the dynamic response versus time for the

,

sensor.

,

acts as electron donor [33], transferring electrons to the graphene film, followed by 18

Fig. 7. (a) The schematic diagram of PECVD graphene-based gas sensor. The optical photograph of interdigitated electrodes (IDEs) is shown in the bottom right corner. The spacing is 100 μm and the scale bar is 300 μm. (b) Raman spectra of two PECVD grown graphene samples. (c) Dynamic response versus time for the sensor. The grey and white regions refer to the injection of air and the pumping down process, respectively. (d) Absolute value of the response with respect to

concentration. (e) Dynamic response versus time for the

sensor. (f) Absolute value of the response with respect to

,

,

concentration.

decreasing the hole concentration of graphene and hence increasing the resistance. The responses of

,

sensing are approximate 2% under each concentration, as

shown in Fig. 7(f). The relatively poor sensitivity of attributed to the low adsorption energy of

,

,

compared to

is

on defective graphene („dn =

−0.24 Gi), as shown in previous density functional theory calculation studies [32]. It is concluded that the PECVD grown graphene strongly interacts with weakly with

,,

which has a potential for ppb-level

4. Conclusion 19

gas sensor.

but

In summary, we present study on the characteristics of highly defective graphene grown through low power CCP-RF-PECVD. It is found that the intensity ratio of D to G band, as well as the 2D to G band, as well as the mobility were strongly affected by the argon and hydrogen flow rates used in the growth process. It is concluded that the defects found in the graphene films are mostly originated from ion bombardment during the growth processes. The vacancy defects on the graphene film provides adsorption sites for gaseous molecules and induces changes of charge transfer in the graphene film. The response of the graphene-based sensor fabricated with the PECVD grown graphene is found to be about 6 % under 100 ppb of

.

Acknowledgement

This work is supported by the Ministry of Science and Technology of Taiwan under contract MOST 106-2112-M008-003-MY3 and MOST 107-2813-C-008-008-M. Reference [1]

K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric Field Effect in Atomically Thin Carbon Films, Science. 306 (2004) 666–669.

[2]

A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (2007) 183– 191.

[3]

C.Lee, X.Wei, J.W.Kysar, J.Hone, Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene, Science. 321 (2008) 385–388. 20

[4]

K.S.Novoselov, A.K.Geim, F.Guinea, N.M.R.Peres, A.H.Castro Neto, The electronic properties of graphene, Rev. Mod. Phys. 81 (2009) 109–162.

[5]

T.O.Terasawa, K.Saiki, Growth of graphene on Cu by plasma enhanced chemical vapor deposition, Carbon. 50 (2012) 869–874.

[6]

Y.S.Kim, J.H.Lee, Y.D.Kim, S.-K.Jerng, K.Joo, E.Kim, J.Jung, E.Yoon, Y.D.Park, S.Seo, S.-H.Chun, Methane as an effective hydrogen source for single-layer graphene synthesis on Cu foil by plasma enhanced chemical vapor deposition, Nanoscale. 5 (2013) 1221–1226.

[7]

D.A.Boyd, W.-H.Lin, C.-C.Hsu, M.L.Teague, C.-C.Chen, Y.-Y.Lo, W.-Y.Chan, W.-B.Su, T.-C.Cheng, C.-S.Chang, C.-I.Wu, N.-C.Yeh, Single-step deposition of high-mobility graphene at reduced temperatures, Nat. Commun. 6 (2015) 1–8.

[8]

C.-C.Yen, Y.-C.Chang, H.-C.Tsai, W.-Y.Woon, Nucleation and growth dynamics of graphene grown through low power capacitive coupled radio frequency plasma enhanced chemical vapor deposition, Carbon. 154 (2019) 420–427.

[9]

Z.Bo, Y.Yang, J.Chen, K.Yu, J.Yan, K.Cen, Plasma-enhanced chemical vapor deposition synthesis of vertically oriented graphene nanosheets, Nanoscale. 5 (2013) 5180–5204.

[10] T.Petrova, E.Benova, G.Petrov, I.Zhelyazkov, Self-consistent axial modeling of surface-wave-produced discharges at low and intermediate pressures, Phys. Rev. E. 60 (1999) 875–886. [11] I.Vlassiouk, M.Regmi, P.Fulvio, S.Dai, P.Datskos, G.Eres, S.Smirnov, Role of hydrogen in chemical vapor deposition growth of large single-crystal graphene, ACS Nano. 5 (2011) 6069–6076. [12] J.L.Qi, W.T.Zheng, X.H.Zheng, X.Wang, H.W.Tian, Relatively low 21

temperature synthesis of graphene by radio frequency plasma enhanced chemical vapor deposition, Appl. Surf. Sci. 257 (2011) 6531–6534. [13] J.Kim, M.Ishihara, Y.Koga, K.Tsugawa, M.Hasegawa, S.Iijima, Low-temperature synthesis of large-area graphene-based transparent conductive films using surface wave plasma chemical vapor deposition, Appl. Phys. Lett. 98 (2011) 96–99. [14] N.Woehrl, O.Ochedowski, S.Gottlieb, K.Shibasaki, S.Schulz, Plasma-enhanced chemical vapor deposition of graphene on copper substrates, AIP Adv. 4 (2014) 1–9. [15] L.Gao, W.Ren, H.Xu, L.Jin, Z.Wang, T.Ma, L.P.Ma, Z.Zhang, Q.Fu, L.M.Peng, X.Bao, H.M.Cheng, Repeated growth and bubbling transfer of graphene with millimetre-size single-crystal grains using platinum, Nat. Commun. 3 (2012) 1– 7. [16] C.-M.Yang, T.-C.Chen, Y.-C.Yang, M.Meyyappan, Annealing effect on UV-illuminated recovery in gas response of graphene-based NO2 sensors, RSC Adv. 9 (2019) 23343–23351. [17] M.M.Lucchese, F.Stavale, E.H.M.Ferreira, C.Vilani, M.V.O.Moutinho, R.B.Capaz, C.A.Achete, A.Jorio, Quantifying ion-induced defects and Raman relaxation length in graphene, Carbon. 48 (2010) 1592–1597. [18] L.G.Cançado, A.Jorio, E.H.M.M.Ferreira, F.Stavale, C.A.Achete, R.B.Capaz, M.V.O.O.Moutinho, A.Lombardo, T.S.Kulmala, A.C.Ferrari, L.G.Cançado, A.Jorio, E.H.M.M.Ferreira, F.Stavale, C.A.Achete, R.B.Capaz, M.V.O.O.Moutinho, A.Lombardo, T.S.Kulmala, A.C.Ferrari, Quantifying Defects in Graphene via Raman Spectroscopy at Different Excitation Energies, Nano Lett. 11 (2011) 3190–3196. [19] A.Eckmann, A.Felten, A.Mishchenko, L.Britnell, R.Krupke, K.S.Novoselov, 22

C.Casiraghi, Probing the nature of defects in graphene by Raman spectroscopy, Nano Lett. 12 (2012) 3925–3930. [20] A.Eckmann, A.Felten, I.Verzhbitskiy, R.Davey, C.Casiraghi, Raman study on defective graphene: Effect of the excitation energy, type, and amount of defects, Phys. Rev. B. 88 (2013) 1–11. [21] A.Mohanta, B.Lanfant, M.Asfaha, M.Leparoux, Methane dissociation process in inductively coupled Ar/H2/CH4 plasma for graphene nano-flakes production, Appl. Phys. Lett. 110 (2017) 1–5. [22] R.F.G.Meulenbroeks, A.J.VanBeek, A.J.G.VanHelvoort, M.C.M.Van DeSanden, D.C.Schram, Argon-hydrogen plasma jet investigated by active and passive spectroscopic means, Phys. Rev. E. 49 (1994) 4397–4406. [23] F.Tuinstra, J.L.Koenig, Raman Spectrum of Graphite, J. Chem. Phys. 53 (1970) 1126–1130. [24] B.Jackson, M.Persson, A quantum mechanical study of recombinative desorption of atomic hydrogen on a metal surface, J. Chem. Phys. 96 (1992) 2378–2386. [25] J.Chen, T.Shi, T.Cai, T.Xu, L.Sun, X.Wu, D.Yu, Self healing of defected graphene, Appl. Phys. Lett. 102 (2013) 1–6. [26] B.An, S.Fukuyama, K.Yokogawa, M.Yoshimura, Evolution of Ar+-damaged graphite surface during annealing as investigated by scanning probe microscopy, J. Appl. Phys. 92 (2002) 2317–2322. [27] Y.Ahn, J.Kim, S.Ganorkar, Y.H.Kim, S.IlKim, Thermal annealing of graphene to remove polymer residues, Mater. Express. 6 (2016) 69–76. [28] E.W.Hill, K.S.Novoselov, A.K.Geim, M.I.Katsnelson, P.Blake, S.V.Morozov, F.Schedin, Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (2007) 652–655. 23

[29] Z.H.Ni, H.M.Wang, Z.Q.Luo, Y.Y.Wang, T.Yu, Y.H.Wu, Z.X.Shen, The effect of vacuum annealing on graphene, J. Raman Spectrosc. 41 (2010) 479– 483. [30] J.E.Lee, G.Ahn, J.Shim, Y.S.Lee, S.Ryu, Optical separation of mechanical strain from charge doping in graphene, Nat. Commun. 3 (2012) 1–8. [31] S.M.Hussein, I.F.Crowe, N.Clark, M.Milosevic, A.Vijayaraghavan, F.Y.Gardes, G.Z.Mashanovich, M.P.Halsall, Raman Mapping Analysis of Graphene-Integrated Silicon Micro-Ring Resonators, Nanoscale Res. Lett. 12 (2017) 1–8. [32] X.Y.Liu, J.M.Zhang, K.W.Xu, V.Ji, Improving gas sensing properties of graphene by introducing dopant and defect: A first-principles study, Nanotechnology. 20 (2009) 188504. [33] O.Leenaerts, B.Partoens, F.M.Peeters, Adsorption of H2O, NH3, CO, NO2, and NO on graphene: A first-principles study, Phys. Rev. B. 77 (2008) 1–6. [34] X.Lin, J.Ni, C.Fang, Adsorption capacity of H2O, NH3 , CO, and NO2 on the pristine graphene, J. Appl. Phys. 113 (2013) 1–6.

24

Credit author statement Yu-Cheng Chang: Investigation, Software, Writing- Original draft preparation. Chun-Chieh Yen: Investigation. Hung-Chieh Tsai: Resources. Tsung Cheng Chen: Investigation, Resources. Chia-Ming Yang: Resources. Chia-Hao Chen: Resources. Wei-Yen Woon: Conceptualization, Methodology, Validation, Spervision, Funding acquisition, Writing- Reviewing and Editing.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: