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Amination of diamond film by ammonia microwave plasma treatment

J.J. Wei, J.L. Liu, L.X. Chen, L.F. Hei, F.X. Lv, Ch.M. Li

PII: S0925-9635(14)00208-8DOI: doi: 10.1016/j.diamond.2014.10.014Reference: DIAMAT 6330

To appear in: Diamond & Related Materials

Please cite this article as: J.J. Wei, J.L. Liu, L.X. Chen, L.F. Hei, F.X. Lv, Ch.M. Li,Amination of diamond film by ammonia microwave plasma treatment, Diamond & RelatedMaterials (2014), doi: 10.1016/j.diamond.2014.10.014

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Amination of diamond film by ammonia microwave

plasma treatment

J.J. Wei, J.L. Liu, L.X. Chen, L.F. Hei, F.X. Lv, Ch.M. Li*

Institute of Advanced Materials and Technology, University of Science and Technology

Beijing, Beijing 100083, China

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Abstract

Functionalization of diamond films is important for various applications, such

as electrochemical sensors. In this study, high quality diamond films were prepared by

microwave plasma chemical vapor deposition (MPCVD). These surfaces were then

functionalized by a surface amination modification (ammonia plasma) using the same

system. The characterization results demonstrated that both the surface morphology

and microstructure of the diamond films were not altered by the ammonia plasma

treatment. The surface nitrogen content was assessed by XPS analyses, which

revealed that amine groups (-NH2) were generated on the diamond film surface

efficiently. The -NH2 concentration on modified diamond film surface was equal to

6.71% (denote as -NH2/100C). The contact angle of water was decreased as the

hydrophilicity of the aminated diamond film was increased. Based on the optical

emission spectrum (OES) study, ·NH and ·NH2 radicals were generated in the

microwave plasma, and are regarded as the crucial precursors for creating the amine

group on diamond surface.

Keywords: diamond film; microwave plasma treatment; ammonia; OES; modification

mechanism

1. Introduction

The electrochemical detection of bio-molecules requires electrode materials with

specific properties such as biocompatibility, stability, reproducibility and sensitivity [1-3]

. Recently, diamond film has attracted much attention and been recognized as one

of the best electrochemical sensor materials due to its unique physical and chemical

properties [4,5]

. The diamond film surface is chemically stable, exhibits favorable

biocompatibility and shows an enlarged potential window together with a low

background current, as compared to other sensor electrode materials such as silicon,

gold or glassy carbon [6,7]

. The immobilization of DNA molecules on diamond surface

shows a higher stability as compared to other conventional substrates [8]

. This novel

DNA biosensor combines the outstanding electrochemical properties of diamond as a

transducer with the controlled bonding of DNA molecules.

However, as-deposited diamond film is considered chemically inert to most

reagents and its chemical modification is straightforward. During the last decade,

progress has been made on the development of easy, controllable and specific surface

modification methods for introduction of different functional groups on the diamond

surface. These methods are based on chemical, photochemical, electrochemical and

physical concepts. Among them, amine group modification is proved to be an efficient

functionalization of diamond surface for the application of biosensor [9,10]

. Such as

covalent bonding of amine terminated alkyl chains [11,12]

, UV irradiation in ammonia

gas [13]

, and the use of radio frequency plasmas of NH3 [14]

.

To improve the detection efficiency, methods to achieve higher coverage of

amine groups on the diamond film surface are needed. As for the aforementioned

amination methods, the highest typical N concentration on diamond surfaces (H or O

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terminated diamond film) is 7%, with only a portion of these available for

bio-functionalization[2]

. Moreover, some of these amination methods are challenging

to apply due to the equipment needed or the complex chemical reactions.

In this study, an ammonia microwave plasma treatment was performed to

introduce the amine groups on the as-deposited diamond film surface. Due to the

higher substrate temperature in contrast to other modification methods and the higher

concentration of ·NH (or ·NH2) radicals in the plasma, greater surface amination can

be achieved. The modification mechanism is proposed based on the OES

measurements of reactive species in the plasma and theory analysis.

2. Experimental

The diamond films were deposited on silicon substrates (10×10 mm) using a

microwave plasma chemical vapor deposition (MPCVD) system with an output

frequency of 2.45 GHz. The substrates were prepared by first dry abrading with

diamond powder (10 μm diameter，bought from Polaris Company, China) and then

cleaned ultrasonically in acetone solution for 15 min. CH4 and H2 were fed into the

chamber at a total reactor pressure of 8.0 kPa. The temperature of the substrate was

kept at 850 ℃. After a 10-h film deposition, the CH4 gas was turned off, and the H2

flow was maintained until the sample was cooled down to the room temperature or the

needed temperature for amination process. The diamond film was grown with the

average thickness 5 μm. Untreated diamond film was denoted as “as-deposited

diamond film”.

To aminate diamond film, pure NH3 gas (99.9%) was supplied at the end of the

deposition, and the H2 flow rate was reduced to 50 sccm to keep the plasma stable.

Meanwhile, the input microwave power was reduced to 600W and the chamber

pressure was lowered to 2.5 kPa. After 15 min, shut down the microwave plasma

treatment and leave the diamond film to cool down at room temperature in an

ammonia flux. The diamond film after ammonia microwave plasma treatment was

denoted as “amination diamond film”.

The detailed deposition and amination parameters are shown in Table 1.

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The diamond film morphology and microstructure were characterized before and

after the ammonia plasma treatment by scanning electron microscopy (SEM,

LEO-1450) and micro-Raman spectroscopy (JY-H2800, 532 nm, 3 mW). The

information of element and/or group on surface of the as-deposited and amination

diamond films was determined by XPS (AXIS ULTRA, Al Kα=1486.6eV). Amine

group concentration was referred with respect to a carbon concentration in percents

and denoted as -NH2/100C or amine efficiency. Amine selectivity denoted as

-NH2/100N ratio gives information about selectivity of the process with respect to

amine group. Active radicals in the plasma were detected by OES (71MS3011). Water

contact angles of diamond films before and after amination process were measured by

Contact measure instrument (Kruss DSA100) to evaluate the diamond surface activity

briefly.

3. Results and discussion

3.1 Film quality

The morphology of diamond films before and after ammonia microwave plasma

treatment are showed in Fig.1a and Fig.1b, respectively. For the aminated diamond

film, the NH3 flow rate was 80sccm with 600W microwave power input and 15 min

treatment time. As seen in these two figures, the diamond film consists of many well

facetted diamond grains with an average grain size of 2 μm. Clearly, the film

morphology is unaffected by the NH3 microwave plasma treatment.

The microstructure of the diamond films before and after ammonia plasma

treatment was studied by Raman spectroscopy. Characteristic spectra are presented in

Figure 2. One sharp peak at 1332 cm-1

due to sp3 C-C bond can be observed in

as-deposited diamond film, which implies that a high quality diamond film was

deposited on silicon by MPCVD. After amination for 15 min, the peak in 1500 cm-1

is

increased slightly, which originates from an increase in the nondiamond carbon

content on the surface [15]

. For the most part, however, the microstructure of the

aminated diamond was not significantly affected by the treatment.

3.2 Surface Group Analysis

To characterize the changes in the atomic composition on the diamond film after

aminated at various NH3 flow rates, XPS analysis was conducted. These aminatied

diamond films were treated at various NH3 flow rates (20, 40, 60, 80sccm,

respectively) and 50sccm H2 gas flow rate, a microwave power of 600W and pressure

of 2.5 kPa, time of 15 min. The results showed that increased ammonia flow produces

increased nitrogen coverage based on the increased N1s core level intensity near 402

eV. Not only the peak intensity increased, but also the binding energy shifts from

401 to 399 eV.

For functionalization, not all of the incorporated nitrogen can be linked to

biomolecules. It is the primary amines that are most active. According to previous

literatures [2,16]

, the N 1s peak at 399eV can be ascribed to “C-NH2” termination,

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which is especially important for bio-functionalization. The N 1s peak at 401eV is

less active and less desirable for biosensors.The signal at this position can be assigned

to nitrogen atoms linked to 2 carbon atoms, as double carbon-nitrogen bonds “C=N”

or bridges “C-N-C”. The results indicate that more primary amines are incorporated

with increasing ammonia flow rate.

Concerning the peak intensities of the two components –NH2 and –N=C (or

C-N-C), their proportion is obviously affected by the NH3 flow rate. To calculate the

relative levels of these two functional groups , the total N 1s peak is separated as two

peaks, one for –NH2 situated at 399eV, another for –N=C situated at 401eV. Take the

as-deposited diamond film and 80sccm NH3 flow rate microwave plasma treated

diamond film for examples, the results of peak separation are shown in Fig. 3. As for

as-deposited diamond film, almost 100% of the N 1s was detected at the 401eV

binding energy. That means the diamond film without any modification will be of less

activity and hardly be used as the substrate material of electrochemical biosensor.

After the diamond film was treated by ammonia plasma, with the NH3 flow rate of

80sccm, microwave plasma of 600w, duration time of 15min, the binding energy of N

1s peak will shift to low energy and the intensity will be enhanced obviously. More

than 95% of N 1s are existed as the type of -C-NH2.

In order to determine the concentration of main elements on the diamond films,

the elemental analysis of as-deposited diamond film and aminated diamond films at

various NH3 flow rates were discussed and the results were listed in Table 2.

From Table 2, the trend of the main element or group concentration as the

function of NH3 gas flow rate can be drawn as fig. 4. As shown in this figure, the total

N atom concentration on the diamond surface increases with rising NH3 gas flow rate,

from 0.81% for as-deposited diamond film to 6.93% for aminated diamond film under

80sccm NH3 flow rate. By the separation of N 1s spectra peak, the concentration of

–NH2 group was also increasing with rising the NH3 flow rate. The trend of O atom

concentration as a function of NH3 flow was irregular. Because the atmosphere of film

deposition and amination has almost no oxygen atom, the detected O atoms on the

surface mainly originated from the adsorption of oxygen when the sample exposed in

the air. It is affected significantly by not only the activity of diamond film surface, but

also the time that diamond film exposed to the air. Therefore the result of O 1s

detection will not be the direct evidence that the activity of diamond film surface was

improved. To explain the actual functional efficiency, the concentration of amine

group is needed to be quantified.

The concentration of amine group is smoothly increasing with rising NH3 flow

rate. It means that the microwave plasma treatment introduces the –NH2 group on the

diamond film surface successfully. Moreover, the trend line of –NH2 concentration is

approaching gradually to the trend line of the total N 1s concentration. It demonstrates

that the main state of N 1s on the diamond surface is transforming from the type of

C=N (little or no bio-activity) to –NH2 (well bio-functionalzation) as the NH3 flow

rate increase. The trend of –NH2/100N was shown in Fig.5. Without any treatment,

as-deposited diamond film has almost no amine group on its surface and hardly to be

used as the substrate of electrochemical sensor. After ammonia microwave plasma

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treatment, the amine group appeared on the surface, and its concentration increased

with rising NH3 flow rate. The proportion of -NH2 group in the total N 1s increases

from 0 to over 95%, which means that the NH3 microwave plasma can reach a high

amination efficiency comparing with conventional amination methods, such as

UV-irradiation or radiofrequency plasma treatment. The -NH2 concentration on the

surface treated by the 80sccm NH3 flow rate was equal to 6.71% (denote as

-NH2/100C), which reached a value close to the maximum obtained with other direct

amination methods [2]

. It proves that the NH3 microwave plasma treatment is not only

an easy-operated amination way, but also an effective method to produce amine-group

at diamond surface.

3.3 Surface Wettability

The surface wettability of the aminated diamond film was compared with that

of the hydrogen-terminated film using water. The average contact angle for the

as-deposited, hydrogen-terminated film was 99°. After amination, the water contact

angle decreased to 55° reflecting greater hydrophilicity. Actually, the contact angle

show here is the average value for 5 measurements, and the standard deviation value

is no more than 3 degree.

It is confirmed that both micrograph and microstructure of diamond film were

not deteriorated by the ammonia microwave plasma treatment. Therefore it can be

conclude that the improvement of surface activity was mainly due to the reason of

hydrophilicity character of amination diamond film. This characteristic will enhance

the adsorption of active bio-molecule on the solution, and then make the aminated

diamond film be a promising biosensor material.

3.4 Modification Mechanism

In order to certify the modification mechanism during the plasma treatment,

optical emission spectrum was adopted to detect the active radicals in the plasma.

Usually, the surface temperature and the plasma contents acted as the crucial factors

for the treatment. The reason why we used ammonia instead of N2 as the plasma

activated source is that the NH3 plasma makes it easier to obtain more active radicals

(eg. ·NH2, ·NH), which will react with the hydrogen terminated diamond surface

(H-diamond). Fig.6a showed the OES results of the NH3 and N2 plasma during the

microwave activation process, respectively. For a better recognition, the spectrum is

limited to the species of interest in the 300 nm to 450 nm range. The power of 800W

and the substrate temperature of 350 ℃ were kept respectively. The most

significant ·NH radical line in our experimental conditions is the 336.0 nm line, which

corresponds to the excitation of the NH3 and will be beneficial to the formation of

aminated diamond surface. However, when the N2 gas was adopted, little ·NH was

observed in the plasma.

The high power will be harmful to the formation of NH2 termination on diamond

surface, due to the shortage of the ·NH radicals in the plasma at the high power.

Fig.6b shows the comparison of the OES results between the NH3 and N2 plasma at

1600 w. It can be found that these two curves were extremely similar. Because the

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atom ratio of N/H were kept at the same value in these two system, the results

demonstrated that under the relatively high power input, it is hard to harvest the ·NH

radicals, which are crucial radicals to form the aminate diamond surface.

The modification mechanism of the amination process by ammonia microwave

plasma treatment can be described in fig. 7. The addition of H2 gas was to make the

plasma stable. At the relatively low input microwave power, the NH3 molecule will be

excited and decomposed into many products (e.g. ·NH2, ·NH, ·N, ·H) (Eq. 1). Among

these radical, ·NH and ·NH2 are regarded as the crucial products to create the amine

group on the diamond film by interacting with the non-activated and activated sites on

the as-deposited diamond film surface [2,17,18]

.

Normally, the surface of microwave plasma CVD diamond film contain

hydrogen termination (H-diamond) and radical site (·-diamond).The ·NH radical can

abstract hydrogen from the hydrogen termination diamond interface forming amine

group (Eq. 2). Similarly, the formed ·NH2 radical can combined a radical site on the

diamond surface to form amine group (Eq. 3). When the ·NH2 group abstract the

hydrogen termination, an ammonia and radical site were reproduced (Eq. 4). Very

similar mechanism was deduced by Wang and co-authors [19]

.

NH3+H2 ·NH2+·NH+·N+·H+e (1)

H-diamond+·NH diamond-NH2 (2)

·-diamond+·NH2 diamond-NH2 (3)

H-diamond+·NH2 ·-diamond+NH3 (4)

Base on the aforementioned results, the crucial radicals which will contribute to

aminated diamond surface are ·NH and·NH2. During ammonia microwave plasma

treatment process, the content of various radicals are affected obviously by the input

microwave power. When the microwave power is too high, the main radical in NH3

plasma is ·N or ·N2 radicals [20]

, which is useless to form amine group on the diamond

surface [21,22]

. To form enough ·NH and/or ·NH2 radicals, it is necessary to optimize

the input microwave power. Therefore, keeping the microwave plasma treatment

process in a moderate microwave power may be beneficial to obtaining aminated

diamond film successfully.

4. Conclusion

High quality diamond film was deposited on silicon by microwave plasma CVD.

By altering the gas composition after film deposition and adjusting the parameters of

the MPCVD system, amination of the diamond film was realized. Microwave plasma

treatment was used as the amination method. This method is not only convenient (the

deposition and the amination process are accompanied in the same system), but it is

also an efficient way of producing a relatively high coverage of primary amines on the

surface. The surface morphology and microstructure were not affected by the

ammonia plasma treatment. XPS revealed that the state of N atom on the diamond

surface gradually transformed from the -N=C to –NH2 group with increasing

ammonia flow rate. After amination using 80sccm NH3, the ratio of –NH2/100N was

more than 95%, which means most of N on the diamond surface was useful (e.g.,

Microwave plasma

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primary amine) for further biofunctionalization. After amination, the water contact

angle decreased from 99° to 55° due to the increased hydrophilicity of the modified

diamond surface as compared to the hydrogen-terminated surface after growth. From

the OES measurements, it can be concluded that ·NH and·NH2 radicals were

generated in the plasma. We suppose these are the primary precursors for amine

functionalization.

Acknowledgments

This work was sponsored by the Fundamental Research Funds for Central

Universities (No. FRF-TP-13-035A), the National Natural Science Foundation of

China (NSFC) (No.51272024), and the Ph.D. Programs Foundation of Ministry of

Education of China (No.20110006110011).

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Figure Captions:

Fig.1. SEM morphology of diamond film before (a) and after amination(b)

Fig.2. Raman spectra of diamond films before and after amination

Fig.3. High resolution N1s spectra for diamond films before and after NH3 plasma

treatment with 600 microwave power and 15 min at a NH3 gas flow rate of 80sccm

Fig.4. Element and group content of diamond film surface before and after NH3

plasma treatment with 600 microwave power and 15 min at a NH3 gas flow rate of

80sccm

Fig.5. Amination efficiency of diamond films before and after NH3 plasma treatment

with 600w microwave power and 15 min at a NH3 gas flow rate of 80sccm

Fig.6. Optic Emission Spectrum of plasma: (a) Emission spectra of NH3 and N2 under

800W; (b) Emission spectra of NH3 and N2 under 1600W

Fig.7. Schematic illustration of the amination process of diamond film by NH3

microwave plasma treatment

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Table caption:

Table 1-Diamond films deposition and amination parameters.

Table 2-XPS elemental analysis of the plasma modified diamond films as a function

of NH3 flow rate for 15 min at a microwave power of 600W.

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Fig.1a

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Fig. 1b

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Fig.2

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Fig.3

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Fig.4

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Fig.5

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Fig. 6a

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Fig. 6b

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Fig.7

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Table 1 Diamond films deposition and amination parameters.

Experimental parameters

Deposition Amination

Microwave power (W) 1800 600

Pressure (kPa) 8.0 2.5

Substrate temperature (℃) 850 300

Hydrogen flow rate (sccm) 200 50

Methane flow rate (sccm) 3 0

Ammonia flow rate (sccm) 0 20/40/60/80

Duration time (min) 600 15

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Table 2-XPS elemental analysis of the plasma modified diamond films as a function

of NH3 flow rate for 15 min at a microwave power of 600W.

Diamond film N(atom.conc. %) O(atom. conc. %) -NH2/100C -NH2/100N

As-deposited 0.81 8.36 0 0

20sccm NH3 4.83 9.22 1.90 39.24

40sccm NH3 5.60 5.53 4.31 76.93

60sccm NH3 5.92 10.38 4.63 78.09

80sccm NH3 6.93 6.26 6.71 96.09

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Diamond and Related Materials

PRIME NOVELTY Statement

In this study, microwave-assisted plasma was used to aminate the diamond film surface. The

process produced a higher surface nitrogen coverage as high as 6.71% than conventional RF treatment and

the process is easier to apply. In this article, both experiment process and modification mechanism are

showed in detail.

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Highlights

For this manuscript, the highlights are summarized as follows:

1) The deposition and amination of diamond film were carried out in the same

MPCVD system.

2) The as-deposited MPCVD diamond film was treated by Microwave-excited

ammonia plasma, which is demonstrated as an effective method to modify

diamond surface with amino group.

3) The modification mechanism was discussed briefly and verified by theory analysis

and OES study.