Amination of diamond film by ammonia microwave plasma treatment

    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: DOI: Reference:

S0925-9635(14)00208-8 doi: 10.1016/j.diamond.2014.10.014 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 & Related Materials (2014), doi: 10.1016/j.diamond.2014.10.014

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ACCEPTED MANUSCRIPT Amination of diamond film by ammonia microwave plasma treatment

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

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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.

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

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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.

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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.

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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.

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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.

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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].

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Microwave plasma 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.,

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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.

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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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[16]S. Torrengo, A. Miotello, L. Minati, I. Bernagozzi, M. Ferrari, M. Dipalo, E. Kohn, G. Speranza, The role of oxygen in the one step amination process of nanocrystalline diamond surface, Diamond Relat. Mater. 20 (2011) 990-994. [17]Y. Coffinier, S. Szunerits, C. Jama, R. Desmet, O. Melnyk, B. Marcus, L. Gengembre, E. Payen, D. Delabouglise, R. Boukherroub, Peptide immobilization on amine-terminated boron-doped diamond surfaces, Langmuir, 23 (2007) 4494-4497. [18]Ch.L. Chen, B. Liang, D. Lu, A. Ogino, X.K. Wang, Amino group introduction onto multiwall carbon nanotubes by NH3/Ar plasma treatment, Carbon 48 (2010) 939-948. [19]M. Wang, N. Simon, G. Charrier, M. Bouttemy, A. Etcheberry, M. Li, R. Boukherroub, S. Szunerits, Distinction between surface hydroxyl and ether groups on boron-doped diamond electrodes using a chemical approach, Electrochem. Commun.12 (2010) 351-354. [20]A. Qayyum, S. Zeb, M.A. Naveed, N.U. Rehman, S.A.Ghauri, M. Zakaullah, Optical emission spectroscopy of Ar–N2 mixture plasma, Journal of Quantitative Spectroscopy and Radiative Transfer, 107(2007) 361-371. [21]T. Vandevelde, T.D. Wu, C. Quaeyhaegens, J. Vlekken, M. Olieslaeger, L. Stals, Correlation between the OES plasma composition and the diamond film properties during microwave PA-CVD with nitrogen addition, Thin Solid Films 340 (1999) 159-163. [22]J.A. Smith, K.N. Rosser, H. Yagi1, M.I. Wallace, P.W. May, M.N.R. Ashfold, Diamond deposition in a DC-arc Jet CVD system:investigations of the effects of nitrogen addition, Diamond Relat. Mater. 10 (2001) 370-375.

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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

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Amination 600 2.5 300 50 0 20/40/60/80 15

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Microwave power (W) Pressure (kPa) Substrate temperature (℃) Hydrogen flow rate (sccm) Methane flow rate (sccm) Ammonia flow rate (sccm) Duration time (min)

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Deposition 1800 8.0 850 200 3 0 600

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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.

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-NH2/100C 0 1.90 4.31 4.63 6.71

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O(atom. conc. %) 8.36 9.22 5.53 10.38 6.26

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N(atom.conc. %) 0.81 4.83 5.60 5.92 6.93

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Diamond film As-deposited 20sccm NH3 40sccm NH3 60sccm NH3 80sccm NH3

-NH2/100N 0 39.24 76.93 78.09 96.09

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PRIME NOVELTY Statement

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

the process is easier to apply.

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process produced a higher surface nitrogen coverage as high as 6.71% than conventional RF treatment and In this article, both experiment process and modification mechanism are

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showed in detail.

ACCEPTED MANUSCRIPT Highlights For this manuscript, the highlights are summarized as follows:

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

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MPCVD system.

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

diamond surface with amino group.

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

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

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and OES study.