β-SiC Composite Thin Films for

Nov 9, 2010 - Institute of Materials Engineering, UniVersity of Siegen, Paul-Bonatz-Str. 9-11, 57076 Siegen, Germany, and. Department of Physical Chem...
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J. Phys. Chem. C 2010, 114, 20207–20212

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Controlled Wettability of Diamond/β-SiC Composite Thin Films for Biosensoric Applications Hao Zhuang,† Bo Song,‡ Vadali V. S. S. Srikanth,†,§ Xin Jiang,*,† and Holger Scho¨nherr‡ Institute of Materials Engineering, UniVersity of Siegen, Paul-Bonatz-Str. 9-11, 57076 Siegen, Germany, and Department of Physical Chemistry, UniVersity of Siegen, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany ReceiVed: September 23, 2010; ReVised Manuscript ReceiVed: October 25, 2010

The wettability of different diamond/β-SiC composite thin film surfaces has been studied before and after chemical treatment by measuring contact angles in both static and dynamic modes; the water contact angle varies from 92.6 ( 2.5° to 32.7 ( 2.8° as the β-SiC content in the composite films (after chemical treatment) increases. The thermodynamic surface tension components of the film surfaces are calculated by using Owens-Wendt-Rabel-Kaelble’s and van Oss’s methods. The increase in hydrophilicity is attributed to the increase in the total surface energy, which is induced by the increase of the Lewis acid-base surface tension component (γAB). The point of zero charge (PZC) of the film surfaces was determined to be at pH 4. Additionally, a composite film surface with a wettability gradient was also fabricated; here, the water contact angle varies from 86.7° at the left edge to 25.1° at the right edge of the film surface. To analyze the molecular scale origin of the observed contact angle variations, time-of-flight secondary ion mass spectrometry was employed to understand the surface termination of the “as-deposited” and “treated” diamond/β-SiC composite films. The controlled wettability was explained by the correlation of the surface termination with the measured contact angle. This work shows that diamond/β-SiC composite thin film surfaces provide varied surface energies that can serve as substrates for diverse biotechnological activities that require different surface wettabilities. 1. Introduction In recent years, the field of biosensors has enormously increased;1-3 during the course of time, various thin film based material systems have been developed for biosensoric applications.4-7 One of the basic requirements of thin film based biosensoric applications is the sustained and long-term functioning of the thin film’s surface under the influence of fluid media.5-7 Therefore, the thin film surfaces are more than often chemically or physically treated to make them clean and highly active,5-13 aiming at enhancing the adhesion of the chemical or biochemical species on the solid surface while minimizing the nonspecific adsorption of the those species, which disturbs the sensing process.12,13 As a result, the wettability of the thin film surface is important to explain its interactions with chemicals or biological species. It has been illustrated that (i) different biotechnological applications require different surface hydrophilicities/hydrophobicities14-20 and (ii) there is a great need to fabricate thin films with the ability to show continuous surface energy (position-dependent wettability) change on the same surface.21-24 The gradient natured surfaces attracted lots of attention because of their applications in the investigation of protein/surface interaction,22 driving flow in microfluidic devices,23 and increasing heating transfer efficiencies in heating/ cooling systems.24 Generally, two methods are used to obtain surfaces with different wettabilities: (i) depositing a self-assembled monolayer (SAM)21 along with changing the surface roughness,25 which can alter the surface properties without changing the bulk * To whom correspondence should be addressed. Tel: +49 271 7402966. Fax: + 49 271 740 2442. E-mail: [email protected]. † Institute of Materials Engineering. ‡ Department of Physical Chemistry. § Current address: School of Engineering Sciences and Technology, University of Hyderabad, Hyderabad, India 500046.

properties, and (ii) fabricating composite film surfaces by using components with different hydrophilicities and obtaining controllable wetting properties.26 Keeping in mind the complications involved in fabricating a SAM layer or designing surface roughness,27 the second method, which will be demonstrated in this paper, becomes very attractive. Following this idea, a diamond/β-SiC composite film system is chosen as the candidate. That is because of the following: (i) Diamond/β-SiC composite films have been successfully deposited since 1992,28 and the diamond/β-SiC ratio can be controlled by changing the gas-phase reactions.29 (ii) Diamond and β-SiC are known for their good biocompatibilities and diverse sensing abilities.30-33 Moreover, both of the surfaces can be photochemically functionalized to obtain bioactive surfaces.6,8,10 As a result, such composite films have a quite high potential in biosensoric application. (iii) The surface terminations of diamond and SiC are quite different. The SiC surface is known for its difficult Htermination. The conventional methods used for diamond (H2 plasma) and Si (HF etching) H-termination do not work on the SiC surface, leaving an OH-terminated surface.34,35 In this regard, it is possible that the selective H-termination of diamond and OH-termination of β-SiC and controllable surface wettability can be achieved through suitable surface treatment. In this paper, diamond/β-SiC films with different wettabilities are fabricated and the surface energy components are investigated to reveal the mechanism of the films’ wetting behavior. This work shows that diamond/β-SiC composite thin film surfaces provide varied surface energies that can serve as substrates for diverse biotechnological activities requiring different surface wettabilities. Additionally, it will be shown that a composite film surface with gradient wettability can also be fabricated; this thin film is fabricated keeping in mind its great potential in microfluidic applications.

10.1021/jp109093h  2010 American Chemical Society Published on Web 11/09/2010

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TABLE 1: Values of the Lifshitz-van der Waals (γLLW), Electron-Acceptor (γL+), Electron-Donor (γL-), and Acid-Base (γLAB) Components of Liquid Surface Tension39 liquid

γLtot

γLLW

γLAB

γL+

γL-

water diiodomethane formamide

72.80 50.80 58.00

21.80 50.80 39.00

51.00 0 19.00

25.50 0 2.28

25.50 0 39.60

2. Experimental Section 2.1. Film Deposition. A microwave plasma-enhanced chemical vapor deposition (MWCVD) technique was used to synthesize diamond/β-SiC composite thin films. P-type (100) Si substrates were used and mechanically abraded by using diamond powder (1 µm) to achieve a high diamond nucleation density. The deposition was carried out at a constant gas pressure of 55 Torr with a microwave power of 2200 W. The flow rates of H2 and CH4 were maintained at 400 and 4 sccm (standard cubic centimeter per minute), respectively, for all the depositions. Tetramethylsilane (TMS) gas additions varied from 0 to 15 sccm in different experiments. With increasing TMS flow, the diamond/β-SiC ratio decreases from diamond dominating to β-SiC dominating (see Figure S1 in the Supporting Information).29 The TMS flow rate, which is indicative of variation in β-SiC content in the composite films, is adopted as a reference in all the discussions. The films were grown for a period of 9 h, and the film thickness is around 1 µm. 2.2. Surface Treatment. After deposition, each composition of the composite films was divided into two groups, which are referred to as “as-deposited” and “treated” samples, respectively. The surface treatment procedures for the two groups are as follows. As-Deposited Samples. Immediately after deposition, the films are washed in an ultrasonic bath of distilled water and ethanol for 10 min each and dried under a N2 flow. Afterward, this group of samples is stored in a vacuum chamber before measurement. Treated Samples. This group of films is, at first, heated at 250 °C for 30 min in an oxidizing mixture of concentrated H2SO4 and KNO3 to oxidize diamond and β-SiC. The samples are then dipped into HF solution (HF/HNO3 ) 1/15) to remove the surface SiO2 layer of SiC, resulting in an OH-terminated surface (the contact angles after such treatment are near 0). Subsequently, the samples are thoroughly washed in distilled water (the water is changed six times, alternately heated, and in an ultrasonic bath) to remove any residual acidic species and are dried in a N2 atmosphere. Subsequently, the samples are treated in the H2 plasma at a gas pressure of 55 Torr, with a microwave power of 2200 W for 20 min. 2.3. Contact Angle Measurement. Static contact angle measurements are performed on the composite film surfaces using the sessile drop method (with an OCA 15plus instrument, Data Physics Instruments GmbH, Germany) at ambient conditions. Three probe liquids have been chosen as the test liquids: water, formamide (99+%, Alfa Aesar), and diiodomethane (99%, Alfa Aesar). Their surface tension components are listed in Table 1. Although several values of Lewis acid and Lewis base surface tension components for water were proposed for calculation, the present study was based on the conventional setting, in which γ+ (water) ) γ- (water) ) 25.5 mJ/m2.36 The reported contact angles are the average of at least three measurements. To investigate the behavior of the composite films’ surface under water flow conditions, dynamic contact angle measurements are carried out on the chemically treated samples. For

dynamic contact angle measurement, a sessile drop was first placed on the film’s surface, and then the syringe needle was dipped into the water drop. To measure the advancing and receding contact angles, the volume of the water was increased and decreased until the three-phase boundary moved outward or inward, respectively. Time-lapse recordings of the drops were captured, and the axisymmetric drop shape analysis-profile (ADSA-P) was analyzed. 2.4. Atomic Force Microscopy (AFM) Measurement. Noncontact AFM (XE-100 instrument, PSIA, Korea) was used to obtain the surface topography and surface roughness information of the composite films. Topography images of the surfaces are obtained with commercial tips with a nominal tip radius of 10 nm. A 5 × 5 µm2 scan with a pixel resolution of 512 × 512 is taken on each sample in order to derive the corresponding root-mean-square (rms) roughness. All scans were acquired at room temperature at atmospheric pressure. 2.5. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) Measurement. The surface termination of the asdeposited and treated samples was examined by using ToFSIMS. The spectra were recorded using a ToF-SIMS IV instrument (ION-TOF GmbH, Germany). Negative secondary ion mass spectra were acquired over the mass range from m/z ) 0 to 100 using Bi+ ions (target current ∼ 1.0 pA). The analysis area for each spectrum was 500 µm × 500 µm, and the acquisition time was set to 30 s. 3. Results and Discussion Figure 1a shows the plot of the water contact angle (CA) on diamond/β-SiC composite films’ surfaces versus the TMS flow used to deposit the composite films. In general, a decrease in the water CA with increasing TMS flow rate has been observed for both as-deposited and treated samples. However, in the case of as-deposited samples, all the measured CAs are above 70°, which shows that the surfaces are relatively hydrophobic. This is expected because only CH4, TMS (both in extremely low concentrations), and H2 (in excess) have been used in the gasphase reaction during the deposition/synthesis procedure of the composite films; this renders the surfaces of the as-deposited samples with H-termination, which turn out to be hydrophobic. As for the treated samples, the water CAs drastically decrease from 92.6 ( 2.5° to 32.7 ( 2.8° with increasing TMS flow rate, indicating an increase in hydrophilicities. In the case of the diamond surface (corresponding to a 0 sccm TMS flow rate), which is considered for comparison purposes, the water CA remains the same even after treatment; this means that the OHterminated diamond surface (due to oxidation in H2SO4) can be fully H-terminated again after the hydrogen plasma treatment. From the above discussion, it can be inferred that the observed decrease in the water CA of the treated diamond/β-SiC composite film surfaces with increasing TMS flow is due to the increase in β-SiC content in the films. To elucidate the surface status of each surface, the negative ion mass spectra (via ToF-SIMS) are recorded and a peak intensity ratio was developed to track the O concentration ([O]/[H] ratio). [O]/[H] represents the relative concentration of O to H in the film and is defined as

[O]/[H] ) IO-related- /IH-relatedwhere

Controlled Wettability of Diamond/β-SiC Thin Films

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Figure 2. Contact angle variations on the composite film surfaces: (a) formamide and (b) diiodomethane.

Figure 1. (a) Water contact angle variation on the composite film surfaces. (b) ToF-SIMS intensity ratio ([O]/[H]) versus TMS flow of the “treated” samples. (c) Contact angle photo illustration along with the surface termination schematic of the “as-deposited” and “treated” composite films.

IO-related- ) IO- + IOH- + ISiO- + ISiOH- + ICOH- + ICOO- + ICOOH- + ISiCOH- + ISiCO- + ISiO-2 IH-related- ) ICH- + ICH2- + IC2H- + IC2H2- + IC3H- + IC3H2- + IC4H- + IC4H2- + ISiHAll intensities (Ix) are obtained from the secondary ion mass spectra. The results are shown in Figure 1b. The [O]/[H] ratios of the treated composite surfaces increase with increasing TMS flow rate, which means that oxygen still bonds with β-SiC even after the H2 plasma treatment. Furthermore, the existence of the small peak corresponding to SiO2- in the SIMS spectra (refer to Figure S2b in the Supporting Information) implies that a small amount of the β-SiC surface was covered by the SiO2 layer. These observations agree well with the previous studies: under the H2 plasma, the OH-terminated β-SiC will only reconstruct to form a well-defined oxidized layer and will not be Hterminated.34 The [O]/[H] ratio, which is 0.3, in the case of the as-deposited surface (a representative sample that is not shown in Figure 1b; a TMS flow rate of 10 sccm is considered), is not so high when compared with the treated sample, but higher than the H-terminated diamond. As H-terminated diamond is quite stable, the slight increase of [O]/[H] in the as-deposited sample against the pure diamond film is mainly due to the slight oxidation of β-SiC in the air during the sample washing and transport, which cannot be avoided (see the presence of the SiO2- peak in Figure S2a in the Supporting Information). This

is also the reason for the slight decrease in the water CA (from 94.1 ( 1.0° to 71.0 ( 1.2°) on the as-deposited samples with increasing TMS flow rate. On the basis of the above observations, the surface termination of the composite films along with the photo illustration of the water CA has been summarized in Figure 1c. Figure 2a,b shows the plots of the formamide and diiodomethane CAs, respectively, on the diamond/β-SiC composite film’s surface versus the TMS flow used to deposit the composite films. Similar to the above-discussed variation of the water CA, the CA variations for the as-deposited samples are not as large as the treated samples, implying the different surface termination between the as-deposited samples and treated samples. The surface energy components of the films are calculated by using Owens-Wendt-Rabel-Kaelble (OWRK)’s37 and van Oss’s38,39 methods. The results are plotted in Figure 3. Although the absolute values of the surface free energy calculated by both methods are different, the observed trends versus the TMS flow rate are the same. As a result, the following discussion is mainly based on the results obtained by the acid-base method (see Figure 3a), which contains more information than the OWRK’s method. As the values of γ- (asdeposited) are close to those of γAB (as-deposited), the plot for γ- (as-deposited) is overlapped by that of γAB (as-deposited). For the same reason, the plot for γ+ (treated) is overlapped by the one of γ+ (as-deposited) in Figure 3a. The total surface free energies (γtot) of the as-deposited composite surfaces stay nearly unchanged, whereas those of the treated samples increase with increasing TMS flow rate. For all surfaces, the Lifshitz-van der Waals components (γLW) are larger than the Lewis acid-base components (γAB). In the case of treated surfaces, the decreasing rate of γLW and increasing rate of γAB are higher than those of the as-deposited surfaces. The increasing rate of γAB is higher than the decreasing rate of γLW in the treated surfaces, which results in an increase in the total surface free energy (γtot). In the Lewis acid-base method, γAB can be expressed as γAB ) 2(γ+γ-)1/2, where γ+ is the part of the surface free energy

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(1) for which, the equilibrium constant KT is given by

KT )

ΓC-O-ΓSi-O-[H+]4 ΓC-OH+2 ΓSi-OH+2

(2)

where ΓC-O-, ΓSi-O-, ΓC-OH2+, and ΓSi-OH2+ are the surface densities of deprotonated and protonated hydroxyl groups, respectively. [H+] is the concentration of protons in solution, which is related to the pH of the probe solution. With pH values higher than the PZC, the majority of the hydroxyl groups exist as C-O- and Si-O-, and the surface is negatively charged. In contrast, when the pH values are lower than the PZC, the majority exist as C-OH2+ and Si-OH2+, and the surface is positively charged. The surface charge density σ can be expressed as12,40 Figure 3. Surface energy components of the composite films calculated by using (a) the acid-base method and (b) OWRK’s method.

resulting from electron-acceptor interactionssLewis acidsand γ- is the part resulting from electron-donor interactionssLewis base. As for the treated samples, γ- increases drastically (from 0.07 to 40.36 mJ/m2) compared with γ+ (from 0.01 to 0.85 mJ/ m2), which induced the dramatic increase of γAB, whereas for the as-deposited sample, γ+ and γ- only meagerly increase (from 0.02 to 1.19 mJ/m2 for γ+ and from 0.003 to 4.59 mJ/m2 for γ-). In general, the acidic component γ+ is given by the contribution of all positive charges resulting from the active site protonation and γ- is given by the negative charge resulting from the deprotonation or the Lewis electron of the active sites.17 In the case of the treated samples, with increasing TMS flow rate, the β-SiC phase that is OH-terminated increases, resulting in the increasing γ- value. However, in the as-deposited samples, the β-SiC is only slightly oxidized; as a result, the γvalue is not as high as in the treated samples. Such an observation compliments well with the above-presented ToFSIMS results (see Figure 1b). It can, therefore, be concluded that the increasing OH-terminated β-SiC phase content increases the hydrophilicities of the composite films. As discussed above, the β-SiC phase in the treated samples is OH-terminated after treatment. Such an OH-terminated surface is charged in solutions with different pH values.12,17 This implication is quite important and has to be taken care of while considering a modified thin film surface for the biosensoric applications. For example, in the case of a diamond thin film based DNA sensor, Kuga et al. found that, with a partially oxidized diamond surface, the diamond surface will be negatively charged at pH ) 7, which significantly reduces the nonspecific adsorption of DNA (negatively charged) onto the surface and increases the biosensitivity.12 To determine the surface charge status of the composite films at a certain pH value, it is important to determine the pH of the point of zero charge (PZC). On the hydroxyl group surface (Si-OH and C-OH), the ionized bonds are present as C-O-, Si-O- or C-OH2+, and Si-OH2+ in solution.12,17 The hydroxyl group acquires an ionic charge, and the overall reaction is as follows12,17

σ/F )

γL d(cos θ) · 2.303RT d(pH)

(3)

where F is the Faraday’s constant, γL is the surface tension of the probe solution, R and T are the gas constant and absolute temperature, respectively, and θ is the contact angle. At the PZC, σ ) 0, which means d(cos θ)/d(pH) ) 0. By plotting cos θ versus pH, the pH of the PZC can be determined. Figure 4 shows a plot of cos θ versus pH for the treated samples. It can be observed that, for all the treated surfaces, the PZC is at pH ∼ 4. This is not surprising because only the OH-terminated β-SiC phase will be influenced by the pH change, and the PZCs of the composite films are determined by the PZC of β-SiC. Our results agree well with the previous studies on the PZC of SiC, which is ranging from pH 2 to pH 4.41,42 At pH ) 7, the composite surfaces will be negatively charged, which will enhance the sensitivity of the DNA sensor.12 On the basis of the above discussions, if one can control the diamond/β-SiC ratio on the same surface, a thin film surface with gradient wettability and, thereby, the surface energy can be fabricated. It is previously proved that a space competition exists between diamond and β-SiC phases during the composite film growth, which leads to the formation of composite films with different diamond/β-SiC ratios.43 Therefore, there are two factors to adjust for controlling the composition of the composite films: (i) TMS flow rate and (ii) diamond nucleation density. In this study, the second method was used. A controlled nucleation density over the surface was achieved during mechanically abrading the substrate with diamond powder. After deposition, the thin film underwent the chemical treatment procedure to obtain OH-terminated β-SiC and H-terminated diamond surfaces. Figure 5a-e shows the SEM plane view images of the composite film surface with a gradient nature from the left edge to the right edge over a length of 15 mm. The bright phase corresponds to diamond, whereas the dark one corresponds to β-SiC. An obvious change in the diamond/βSiC composition can be observed. The left part of the film is dominated by the diamond phase (see Figure 5a), whereas the right edge of the film is dominated by the β-SiC phase (see Figure 5e). The variation of water CA on this surface is shown in Figure 5f. A decrease in the water CA from 86.7° to 25.1° is clearly observed. Such a surface has comparable contact angle

Controlled Wettability of Diamond/β-SiC Thin Films

Figure 4. Plots of cosine of the contact angle on the “treated” composite film surfaces versus pH. The PZCs of the films were pH ∼ 4.

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Figure 6. AFM surface topography images of the “treated” composite film samples: (a) diamond, (b) TMS ) 2.5 sccm, (c) TMS ) 5 sccm, (d) TMS ) 10 sccm, and (e) TMS ) 15 sccm. The rms roughness of each surface is also shown under each image.

hysteresis)47 until the diamond/β-SiC composition reaches 1:1 (in our case, it should be at around TMS ) 5); such a competition results in a decrease of the CA hysteresis. With a further increase of the TMS flow rate, the surface roughness and the heterogeneity both decrease, which induced the CA hysteresis decrease. 4. Conclusion

Figure 5. Surface with gradient hydrophilicities. (a-e) Scanning electron micrographs of the composite film: the surface morphologies of the gradient film from the left edge (high diamond nucleation density) to the right edge (low diamond nucleation density). (f) The contact angle change.

TABLE 2: Dynamic Contact Angle Values (°) on the Treated Composite Film Surfaces TMS flow (sccm)

θa

θr

∆θ

cos(θr) - cos(θa)

0 (diamond) 2.5 5 10 15

94.4 78.7 69.3 50.5 39.5

62.6 44.9 43.2 28.7 21.7

31.8 33.8 26.1 21.8 17.8

0.54 0.51 0.38 0.24 0.16

values as the SAM-modified surfaces,44-46 and it is stable under various environments, which has high potential in biomicrofluid devices and heating/cooling transfer systems. The results pertaining to dynamic contact angle measurements of the treated samples are shown in Table 2. The hysteresis of the CA increases first and then decreases with increasing TMS flow rate. The hysteresis is mainly due to the roughness and heterogeneity of the surface.47,48 The surface topography and rms roughness (Rq) of the composite films as measured by AFM are shown in Figure 6. A decrease in the surface roughness with increasing TMS flow rate can be observed. Although the roughness of the samples is decreasing, the CA hysteresis of the samples increases first (see the ∆θ values in Table 2 for diamond and the TMS ) 2.5 sample), which is due to the increase of the surface heterogeneity. With increasing TMS flow rate, the surface roughness further decreases (decrease the hysteresis)48 while the heterogeneity increases (increase the

The wettability of different diamond/β-SiC composite thin film surfaces before and after chemical treatment was studied by measuring both static and dynamic water contact angles. The contact angles change from 92.6 ( 2.5° to 32.7 ( 2.8° as the β-SiC content in the films increases in the chemically treated samples. Time-of-flight secondary ion mass spectrometry measurement reveals that the β-SiC phase is OH-terminated after the chemical treatment, whereas the diamond is H-terminated. The themodynamic surface tension components of the films are calculated by using Owens-Wendt-Rabel-Kaelble (OWRK)’s and van Oss’s methods. The increase in hydrophilicity is attributed to the increase in the total surface energy, which is induced by the increase of the Lewis acid-base surface tension component (γAB). The PZC of the films, which is at pH∼4, is determined by plotting the cosine of the contact angle versus pH. A composite film surface with a gradient nature was also fabricated by controlling the diamond surface nucleation density; the water contact angles on this surface vary from 86.7° to 25.1°. This work shows that diamond/β-SiC composite thin film surfaces provide varied surface energies that can serve as substrates for diverse biotechnological activities requiring different surface wettabilities. Acknowledgment. We would like to thank Mr. Igor Aronov in the Analytical Chemistry Department, University of Siegen, for the SIMS measurement. We also thank Dr. Yang Nianjun and Dr. Oliver Williams of the Frauhofer Institute for Applied Solid State Physics IAF for the H-termination of certain samples. Supporting Information Available: Surface morphologies of the composite films with a TMS flow from 0 to 15 sccm and the SIMS spectra for the sample with TMS ) 10 sccm are shown in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Scho¨ning, M. J.; Poghossian, A. Analyst 2002, 127, 1137–1151. (2) Daniels, J. S.; Pourmand, N. Electroanalysis 2007, 19, 1239–1257.

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